Driving/low heel recovery issue

I’m having trouble with my block start: I do achieve a triple extension but instead of driving my foot “down and back” my leg “unfolds” at the knee joint (–> bad shin angle for pushing) while my other heel travels a long way up right under the butt. When I try to keep my heel lower then the knee unfolds even more.
Are there any cues that help me avoid that?


Oh, and here’s a video: