Somebody show me when Sergios knee fell apart

[JazzWolf]

Check 1:05 of round one. Martinez twists his right knee and ankle badly as Cotto steps on his front foot and lands a left hook. My knee hurt just watching it.

Edit: If it was from the punch alone, both legs would have gone.

Occurs again at R1 1.25 and 0.43 where Cotto wedges his own knee behind Martinez front knee and pushes him, causing him to collapse over.

Evil as sin but effective tactics.