they didn't let up, they were up 21-0 and went to a more prevent D, letting Michigan run the ball, and took no chance on O
usually PSU has no game plan set on O, they freelance and see what can work, Notre Dame is usually the same way but with more excitement.
up 21-0 unexpectedly (to everyone) let them go into a shell and wait until it got close, then they scored another TD with ease
i don't consider it much ground to praise Michigan that they didn't quit down 21-0
but the dynamics of the game are ruthlessly changed at that point