Originally Posted by NikFights
To break someone's legs backwards like that would need for them to hyper-extend it and then hit it with tremendous force. Supremely unlikely.
Exactly, I'd imagine that throwing said kick with THAT much force would put the person throwing it in considerably more danger than the person he/she was aiming at, because you are coming forward and slightly down to throw an oblique kick with fight ending power, considering they aren't known for their accuracy I would say the person throwing theoretical kick would be in some serious dance of fight ending face destroying counter uppercut. Same reason you don't see a guy throw a teep with the same fight ending commitment as a side kick, it's a rather slow strike, low percentage, you are driving yourself toward your opponent with a hell of a lot of force, and most importantly you are in no position to counter after throwing it so your opponent can simply tee off on you if he reads it.
As Walker Texas Ranger said up there guys have their knees bent as a rule in...every form of fighting ever...so that is what's making it hard you aren't pushing against just the knee ligaments you are trying to push against some of the strongest muscles, tendons, and ligaments in the leg instead.