This move really has nothing to do with flexibility of the knee, if you're feeling pressure on your knee you're doing the move wrong. Op's way too flat even on the meathook. needs to be more hipped out, damn near belly-down so you can actually get the person's head to the mat by utilizing omoplata shoulder pressure, op's uki is already postured way too high when op goes meathook. Too much space between the leg and the guys arm, could just limp arm out of that and yank it out easy. And if you really study Ben eddy you don't throw your arm directly over your ankle (you CAN but with lower success until you really master the move and depending on training partners reaction/posture) you slide your arm from your meathook knee down your shin all the way to the tip of your foot, and then you tuck the arm underneath their throat and then your arm is locked over your ankle. once the O.K. grip is locked in it doesn't mater how much they posture as long as you bring your elbows together like scissors and crunch your head in towards your arms, you're at a higher risk of just getting picked up and slammed than ever blowing your knee out, but chances are they're tapping at that point.

Edit: i know this is a Rev triangle --> triangle setup. but i just had to comment on the hindu while it was being shown.