The definition of slewfooting is really straightforward.
Rule 52 - Slew-footing
52.1 Slew-footing - Slew-footing is the act of a player or goalkeeper using his leg or foot to knock or kick an opponent’s feet from under him, or pushes an opponent’s upper body backward with an arm or elbow, and at the same time with a forward motion of his leg, knocks or kicks the opponent’s feet from under him, causing him to fall violently to the ice.
52.2 Match Penalty - Any player who is guilty of slew-footing shall be assessed a match penalty.
52.3 Fines and Suspensions - There are no specified fines or suspensions for slew-footing, however, supplementary discipline can be applied by the Commissioner at his discretion (refer to Rule 28).
How did any of this not apply at least once to Brad Marchand in this series? He committed a slewfoot on Kris Letang on the first shift of this series, and at least two more clearcut transgressions that I've seen, not behind the play, but right with the puck, where the refs were able to see it.
And for those who say the refs are letting things go on both sides, that's entirely missing the mark. By letting things go, by "letting them play", they're completely favouring the Bruins, this plays right into their hands, and allows them to drag the game into the ditch where they like it. If the refs were calling everything, as the rulebook states they should, the Pens would be allowed to fly, and play the game the way it was intended, with skating and passing and shooting, with an entertaining result for the fans.
If interference and tripping and roughing and aren't really penalties, why have them described as such in the rulebook? If a hooking call in the neutral zone is okay, because it doesn't directly lead to a scoring chance (an asinine distinction, since on the double-overtime goal by Patrice Bergeron, how could it be determined at the time that Jaromir Jagr's hook on Evgeni Malkin wouldn't lead to a goal the other way, which it definitely did), why not just come out and say so, spell it out.
The NHL is the custodian of the game I love, its legal guardian, and I'm this close to initiating legal action to strip it of custody. I've had it with the salary caps and lockouts and insane Mike Milburys and Colin Campbells and Don Cherrys being at the wheel of the ship as it steers full steam ahead toward the iceberg.