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I thought thighs and calves meeting was what created the unhealthy pulling force on the knee.
This is a valid point and there is indeed a pulling force, so I'll try to explain my theoretical understanding of oly squats in more detail.
In both the oly and the PL-style squats, as the lifter approaches maximum depth, all the thigh muscles increase their degree of contraction and, as the muscles get tight, the descent momentum stretches their tendons (some kinetic energy from the descent is stored in the tendons in the form of elastic energy). At the bottom, the muscles are in nearly full contraction and the tendons are stretched (that is why, in order to take full advantage of the stretch reflex, you need to start thinking "PUSH" right before you hit max depth).
Now, if the lifter is doing a PL squat, then the bottom will be just bellow parallel and the tendons will be fully stretched. It follows that there is a specific limit for the speed of descent, and, if that limit is passed, there will be more energy stored in the tendons than they can handle, so the tendons will break.
In an oly squat, there is a certain amount of energy stored in the tendons, and the rest of it is absorbed by "bouncing" your thighs on your calves, so there is no fear of those tendons rupturing (those tendons are basically the hamstrings and adductors). That does produce a pulling force on your knee (like it wants to pull it apart), but at the same time the thigh muscles (specifically the quads) are contracted and their tendons stretched, so that stabilizes the knee joint (it produces an opposite force to the pulling one). In follows that there is also a specific limit to how fast an oly squat can be performed before the pulling force on the knee becomes too great, but that limit is higher than a PL-style squat where the entire force falls on the tendons (mainly) of the hamstrings and the adductors. Simply put, in an oly squat you can handle the same kinetic energy and then some.
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