Well, let's go through the rules first.  I'm going to take it for granted that you understand players on the 40-Man roster are protected.

Really, the biggest and most basic thing you should know is that players 18 or younger (when drafted or signed) are exempt from the Rule 5 draft for five years; players 19 or older are exempt for four years.  There are different costs associated with how the drafting team in the Rule 5 uses their picks, and there are rules about how long players must be active in order to be eligible for the draft, but to begin to scan for players left exposed by the Twins this is all we need for now.

In previous years the Twins have used the Rule 5 draft to grab guys like Ryan-Rowland Smith and Alejandro Machado.  Of course we know the Twins scored big with Johan Santana (in the long long ago of 2000), but thanks to Wikipedia, there are a number of guys who have posted some good (or at the very least, star-crossed and useful) careers after being taken in the Rule 5:  Bobby Bonilla, Shane Mack, George Bell, Dave Hollins, and Willie Hernandez.  More recently, in addition to Santana, Shane Victorino and Josh Hamilton have been selections.

Last week after the Twins made seven more additions to their 40-Man roster, leaving just one open slot, a lot of guys were put in a position of instability.  Minnesota's farm system stocks more talent this off-season than it has in recent memory, and for the first time in as long as I can remember there will be a position player or two that I'm concerned could get taken.

A team can never cover all of its assets, but in one sense it's a good problem to have.  Nevertheless there are a number of prospects exposed, and with the Rule 5 approaching with the Winter Meetings in December, the Twins are leaving some tempting bait on the hook.

Jay Rainville, RHP
2009 Age:  23
Drafted:  1st Round, 2004

Rainville had a good start, rolling through his first three stops in roughly his first 14 months in the Minnesota system.  Baseball American ranked him as the Twins' 8th best prospect going into 2006, which was also the year he lost completely due to surgery on his shoulder.  He had a good comeback year in 2007 in Fort Myers, and after hot start there in '08 was promoted to double-A New Britain.  As a 22-year old there he struggled sometimes, posting a 5.78 ERA in 123 innings, but in addition to his strikeouts dropping and continuing to be a fly-ball pitcher, he also lost a bit of his command.  Rainville doesn't project as a front-line starter by any stretch of the imagination right now, but he's still a very talented arm.  A year ago I might have predicted he'd debut for the Twins sometime late in 2009.  Now I'd probably push that back to 2010, but a strong campaign this next summer could alter that as well.  Jay is just one of the pitchers I'd have preferred to be added to the 40-Man roster in place of Drew Butera.

Erik Lis, 1B/LF
2009 Age:  25
Drafted:  9th Round, 2005

Out of the position players available in the Minnesota farm system, Lis is the one that could be drawing the most interest.  In college he was a hitting machine, and even in his first two stops with the Twins the power was on full display.  In '07 in Fort Myers he had a decent season, and as a result spent all of last summer in New Britain.  He didn't display the power in terms of home runs, but the bigger concern for me was how often he was (or wasn't) getting on base.  While the walk rates continued to drop, the strikeout rates were sneaking higher.  He hit a lot of ground balls this season, and he hasn't been a guy who hits a lot of line drives, either.  Lis did miss the last portion of the season with a shoulder injury, so there is some forgiveness.  But with no defense to speak of, his bat is his ticket to success.  At 24 he wasn't young for double-A, and he didn't exactly rake there, so it's not as though the Twins are exposing a can't-miss prospect...but he's still a hitter, and hitters are always in demand.  I'd still prefer to see what he could do in Rochester.

Kyle Waldrop, RHP
2009 Age:  23
Drafted:  1st Round, 2004

With this move it seems the Twins have given up on the compensatory pick they drafted after losing LaTroy Hawkins.  Waldrop missed all of 2008 with shoulder issues, which means his 59 innings in double-A back in '07 as a 21-year old could be the last vision the organization has of him.  And that vision wasn't really promising.  A 5.34 ERA in those 59 innings, along with 74 hits, is a shame to have as a departing audition because in spite of his mediocre stuff had been effective in prior stops.  He's not a strikeout guy, but when healthy had good control and did a pretty solid job of keeping the ball in the park.  I think it's too early to give up on Waldrop, as I believe his skills could be useful at the back of a rotation or in the bullpen, but I have to think that if he's taken he will simply be a casualty of a farm system that's littered with pitching prospects.

Yohan Pino, RHP
2009 Age:  25
Signed:  Venezuela, 2004

Pino's curious success has hit a stumbling block at double-A.  The strikeouts are down as better hitters aren't as baffled by a sub-par fastball, and he's been getting knocked around a little bit.  Without great stuff and at 25, he'll be too old for any success to really make an imprint unless he would be moved to Rochester next year, but with the Twins constantly moving him from the rotation to the bullpen and back again it's clear that even they don't know what to do with him.  Everything has been stood on its head since his promotion to the Rock Cats, and his exposure doesn't bother me as much as it does with a few of these other pitchers, but I also believe that consistency and confidence can do a lot for a player.  An organization showing a little confidence in a guy like Pino, with unimpressive stuff, could give him the motivation he needs to find the next level.  It's unlikely, but it's not unheard of.

David Winfree, OF
2009 Age:  23
Drafted:  13th Round, 2003

Next to Lis, Winfree is the second position player I can see being taken.  He was shifted to the outfield this year, making room for Luke Hughes and Danny Valencia, but he managed to show improvements.  In his second full season in double-A New Britain, Winfree cut down on the strikeouts, significantly increased how often he walked and added 41 points to his isolated power (.159 to .200)...all in spite of losing 15 points off his batting average, his OPS was up 35 points, to 769.  His lines aren't outstanding, and compared to guys like Matt Tolbert he could come off looking like a younger incarnation of him, but there's some raw power there.  Matt Macri posted a .205 ISO in double-A, but at age 25.  Winfree is three years younger, and he's shown marked improvement from '07 to '08.  In some ways Winfree's progression is still on a clear forward trajectory, and in that way can come off as a bigger oversight than not protecting Erik Lis.  I don't like his exposure at all.

Ryan Mullins, LHP
2009 Age:  25
Drafed:  3rd Round, 2005

Mullins is a southpaw who has a history of keeping the ball on the ground and, more importantly, in the park.  In late 2007 the Twins gave him a shot in triple-A, and he was knocked around in four starts, so they stuck him back with the Rock Cats for the whole of 2008.  He wasn't as good in New Britain his second time around, and at 24 he doesn't have a lot of time to wait around hoping to be promoted, but it's not like he was horrible.  A left-hander with decent velocity who can keep the ball on the ground is the profile of a pitcher that can almost always find work, and I have a hard time believing the Twins simply couldn't find a spot for him.  He's no ace, just like every other pitcher on this list, but it certainly seems the organization has arms of higher priority.  Again:  Drew Butera?

Zach Ward, RHP
2009 Age:  25
Drafted:  3rd Round, 2005

Ward came over from Cincinnati in the Kyle Lohse trade (remember all the Zach Ward 7-0 talk?), and there's one asset that's worth keeping him for:  he's an extreme ground-ball pitcher.  Over his last four stops in the minors, he's induced ground balls anywhere between 55 and 71 percent of the time.  In 367.1 minor league innings he's allowed just 12 home runs.  Control was an issue this season, as he walked about 12% of his hitters, but historically he's never been that erratic, and even though he's not really a strikeout pitcher he still sent down 81 men in 93 innings, and that's not bad.  Right now the only thing working against him is his age, but that could be solved by a promotion to the Rochester bullpen.  If he slips through the drafted unselected, the Twins will have gotten away with one.

Matthew Fox, RHP
2009 Age:  26
Drafted:  1st Round, 2004

Superior strikeout numbers in 67 innings in '04 and '06 sandwiched a lost season in 2005 due to labrum and rotor-cuff surgery.  The result of allowing Eddie Guardado to walk (the first time), Minnesota kept Fox in rookie ball for his return in 2006, and he delivered...big time:  1.12 WHIP, 0.22 HR/9, 10.27 K/9.  The Twins tempered their excitement and moved him up just one level the following year, and shifting between the rotation and the bullpen in Beloit he still posted promising numbers; and he should have, he was 24.  At 25 this last summer he was finally moved to Fort Myers in high-A, and in 14 starts and 18 relief appearances he still struck out 99 in 117.1 innings.  Fox has pitched well, but Minnesota has refused to rush him, and as a result he's far too old for his competition, and therefore it's hard to judge whether or not he's as good as his numbers.  Fox could turn into a solid relief option for someone out there, but it seems like the concerns about his injuries are too much to keep him from being exposed by the Twins.

Tim Lahey, RHP
2009 Age:  27
Drafted:  20th Round, 2004

After being drafted in last year's Rule 5 draft, number one overall, I'll be shocked if Lahey is not taken again.  He's a monster of a man who can induce ground balls, but he struggled a bit in Rochester this last year.  His peripherals insist he was better than his 5.43 ERA, and he missed a few bats, but control has always been an issue with Lahey, walking 3.47 men per nine innings in his minor league career.  Still, he's an arm that some teams (Cubs, Phillies) see as major-league ready, but apparently the Twins are content with their internal options without him.  Lahey is almost certain to be taken.

There are other players available as well:  Juan Portes, Matt Moses.  But those nine above are all players who will definitely be getting some consideration from organizations around the league.  You can never tell when you'll bull a Bonilla or a Santana out of the Rule 5 hat, but whoever we lose will be replaced, probably, by someone we've plucked from another system.  And that's where you hope your external scouting department is better than those who made the internal options on who to protect.