You are doing the right thing having it on your ankle

dont know about this knee buisness

I have never had the problem you are talking about, maybe try jumping to the side when you fall off.
The only problem I ever have is when I stand back up sometimes the legrope is between my back foot and front foot and you have to crouch down and flick it to the back of the board.
Im sure as you get better it will become less of a problem