see linked ND Observer article...scroll to page 4...Tony Capers injury explained.
Reporter Terry O'Neil ( later a famous producer on ABC's Monday Night Football) claimed the Capers injury happened as followed
Capers was in playing defense end...Oklahoma RB Steve Owens(later a Heisman winner) tries to block Tony on a Sooner sweep around DE Capers- but 'misses'...'scrambles on the ground' a few yards & attempts to 'leg whip' Capers in order to take Tony out of the play...misses(again)...but his cleat clips Capers Achilles tendon...snapping the tendon.....Tony Capers season was over
That article gave all the details. I had always thought he got hurt on a kickoff.
Also interesting to read Capers perspective on being a black player/student at ND.
I've linked his Obit. He died too young in 1999:
I recall (don't trust this) that he had his ankle twisted on a tackle in the end zone after scoring a TD before the Liggins play.