They wear down teams, both with depth and with conditioning. Not too many teams can pressure the ball all game long because they aren't in the shape Louisville players are in.
Pitino shed some anecdotal light on some of this the other day when he was discussing how deceptively strong Russ Smith is. The guy is 6'1"-2" and weighs around 140 but can rep 185 on the bench 25 times. Siva and Smith once had a pull-up contest, and Siva made Smith put a 15 pound weight vest on to make it "a little more fair." Siva did 38 pull-ups, Smith did 39.