I film my son a lot & put his swings on my my computer, frame by frame.
The problem is that when he lands, toe & then heel, rather than immediately starting his swing his body will shift forward about 3 frames before he starts his swing. This results in his knee being even more bent by the time he actually starts his swing.
I know you say this forward body shift is bad and I agree with you. The question is HOW to restrain this forward body shift.