I completely agree Rampage here. Attempting to hyper extend someones knee could be career ending. Knees are serious, serious business for athletes. They even made a rule in the NFL now you can't hit Quarterbacks in the knee. That's the money maker right there and those are career ending injuries.
It's not the same as a knee bar where you can tap out. This is a strike that happens instantly.
EDIT: This should NOT be legal.