Yost Ice Arena seemed more like a maximum-security prison than a hockey rink this weekend. Referee Scott Hanson took on the roll of sheriff, as the penalty box stayed packed all weekend long - except for Saturday's third period.

Entering the final stanza, the Wolverines trailed the top-seeded Pioneers 3-2. But with the freedom to play without the fear of being booked and sent to the box, Michigan outscored Denver 3-0 in the period and earned a trip to the Frozen Four.

After calling everything in sight for the first two periods of Saturday's game and all of Friday's game against St. Cloud, Hanson and his crime patrol wouldn't even call blatant penalties on either team in Saturday's third period.

With 11:38 remaining in the contest, Michigan defenseman Eric Werner was punched in the face by Denver forward Matt Weber, and the referee - watching the whole thing unfold - swallowed his whistle.

"I think the referees had to let the teams decide the game," Michigan coach Red Berenson said. "Neither of the teams were taking advantage of the rule book, and some of the penalties that were called were probably marginal at best and weren't fair to the teams that were being punished."

Whether the referees made "marginal" calls or not, it was the timing of Michigan's penalties in the second period Saturday that couldn't have been any worse.

Less than two minutes after Michigan took the lead on forward Eric Nystrom's goal, Michigan defenseman Brandon Rogers was called for a hold. On the ensuing powerplay, Denver forward Chris Paradise tied the game at one.

Once again, after Michigan defenseman Mike Komisarek scored a powerplay goal to give his team a 2-1 lead and all the momentum, Michigan captain Jed Ortmeyer was called for interference, putting Denver on the powerplay again. To make things worse, Komisarek was called for a rare "throwing the puck" penalty moments later to put his team down two men. Denver's Kevin Doell tied the game just as Ortmeyer's penalty expired.

"(The penalties) were huge turning points," Berenson said. "Every penalty was a potential game-winning turning point, and so I was glad when he finally put the whistle away."

Michigan knew that staying out of the box would be a key to its success this weekend - especially against St. Cloud, which boasted the No. 1 powerplay in the country.

But it was more of the same from the Wolverines on Friday. Less than one minute after forward Jason Ryznar scored early in the first period to give Michigan the lead, forward Michael Woodford committed a blatant cross-checking penalty. Thirty seconds later, St. Cloud's Jeff Finger tied the game.

With Michigan leading 3-1 in the third period, Hanson's crew came out with a heavy hand. Michigan center Dwight Helminen was booked for hooking, and 1:30 later, defenseman Jay Vancik was hit with a cross-checking violation. Simultaneously as St. Cloud's 5-on-3 advantage ended, Huskies' forward Jon Cullen scored to bring his team within one goal and put Michigan on the ropes.

"Their powerplay definitely became a factor in the game," Berenson said. "The second period they got a little confidence, a little momentum, and with five consecutive penalties in the third period, their powerplay took over the game. Frankly, we were fortunate to win this game."

"We got called for a couple of penalties, and they had a 5-on3," Michigan freshman forward Milan Gajic said. "All the guys who were out there did an unbelievable job.

"I thought we were going to get through it, because we (had 65 straight penalty kills) during the season. I knew they were going to get the job done."

The Michigan penalty killing unit may have gotten the job done, as it killed off just enough penalties to keep the Wolverines on top in each contest. But Denver and St. Cloud converted four out of their 12 opportunities nonetheless.

"During all that penalty killing in the third period, that's tough," Berenson said. "That's physically and mentally tough. I thought we came through it even though they scored."

The Wolverines' penalty kill has been a strength of the team all season and was a major "source of pride" during their run to the CCHA regular season title. Michigan's 65 straight penalty kills spanned from Dec. 29 to Feb. 23.

The Wolverines' penalty kill has relied on scrappy performances from players like junior J. J. Swistak, who along with Helminen became a professional shot blocker in CCHA play. But against Denver and St. Cloud this weekend, Michigan's forwards had trouble getting in front of shots and seemed to be on their heels throughout each game.

With Minnesota - another explosive WCHA opponent with a superior powerplay - awaiting Michigan in the Frozen Four, the Wolverines' ability to stay out of the box and kill penalties will be a major factor in whether or not they can keep the Gophers' attack grounded.