as far as im aware the manipulation of the joints when doing a crush or slice like the vaporizer is stretching the joint in the opposite direction of things like a armbar, kneebar. instead of making the limb go in a direction it cant normally go you are putting something in the way of where it normally goes to increase the pressure.
the result of this is more often a lot more severe when damage occurs because as far as im aware it snaps things in the joint at a lot more points. youre pushing not only the joint but generally the ligiments and muscles to their utmost limit and then pushing further.
adding a toe hold of some kind which twists the ligiments knee joint and muscles adds even more pressure to it because t applies pressure to the knee at various angles aswell.
the popping sounds alone arnt really all conclusive of a damaged leg. the popping sound is generally gases and the synovial fluid between the joints
aside from the last bit which ive looked up and found scientific articles about before, the rest is just my opinion and im not sure about it.
http://www.loc.gov/rr/scitech/mysteries/joint.html