"Full flexion" refers to the maximum bending or curving of a joint in the body.
Full definition
These 4 ligaments are strongest at full extension (when your legs are fully straight and extended) and
at full flexion which is what happens when you squat below parallel.
It is suggested that this works due to the ability of the exercise to increase the peak eccentric force of the hamstrings at shallower angles of knee flexion (the knee is more extended) vs. a leg curl which puts a premium on concentric force when the knee is
in full flexion.
Just
hitting full flexion or ATG is fine, go any lower (bouncing hard off your calves) and you create a «high wear» situation for your knees.
Although yin work with the knee would
seek full flexion and extension (bending and straightening), it would never aggressively stretch this extremely vulnerable joint.
While you should still break parallel, it's not beneficial to reach the ATG depth like in the high bar squat (where the knees
reach full flexion).
These ligaments are most effective at full extension (when you are standing straight) and
full flexion (when the calves touch your thighs as in the ATG position of a high bar squat).