Michigan has already pulled away late in the fourth quarter, but the starting defensive lineup is still playing. Michigan is in its nickel package, and Minnesota comes out in a four-wide set. As you can see at the top of the screen, the Gophers have three receivers against Michigan's 2.5 defenders (the third is 15 yards from the line of scrimmage). Any offensive coordinator who sees this (and most Michigan fans) would immediately check to a bubble screen. Eight free yards, right?
But when the ball is snapped, rather than rush upfield, Frank Clark immediately looks out to the slot receiver to check if he bows out for the bubble screen.
Before the quarterback can release the ball, Clark is already running toward the slot receiver.
The ball is still int he air and Clark is only 5 yards from the receiver with a full head of steam.
When the receiver finally catches the ball, Clark is in his face. Michigan corners are keeping outside leverage and forcing the ball carrier back inside toward Clark.
Not only does Clark make the TFL, but he also strips the ball carrier in the process, allowing Courtney Avery (currently engaged with MarQueis Gray at the top of the screen) to recover the fumble, which you'll see in the video below.
The obvious question is whether or not this was a designed playcall or just a great play by Clark. The evidence points toward this being something Mattison drew up to entice an offense to throw the bubble screen. First, you can see how immediately Clark looks out to the flat. It may be a check for Clark on this play: if the slot receiver runs vertically, rush the quarterback; otherwise, attack the receiver catching the bubble screen. The other piece of evidence indicating this was a designed playcall is the blitz by Jake Ryan on the weakside of the field and the slant of the defensive tackles. Michigan is still able to rush four in this scenario and are effectively rotating where the rush is coming from. When Clark runs out to the flat, the D-line slants in that direction and Ryan fills the backside with a blitz. The forced fumble and recovery are just gravy.