Do you mean this posed static photo of Riggs ?:
Now imagine it all happens dynamically during the motion as here (do not have time now to upload even more greats):
Do you really believe there is still any room for further "collapsing" of the lead knee towards the rear knee ? I see their lead knee joints meet the natural limitation of bending inside.
Moreover, I see how they all could benefit from such a position later on. I can't see how they could benefit from not letting the knee go and not letting the outer edge of the lead heel lift off.
Cheers