My instructor had me doing something last weekend that helped with this. Different situation - I (think I) have flexed effectively in the past, but since coming back from my knee surgery, I've been guarding the knee and keeping myself from flexing, no matter what I told myself.
I'm not sure I'll communicate this correctly, but maybe
@Skisailor can comment. After watching me a few ski days in a row, my instructor finally had me stand still in my skis and leaaaaan forward until I lifted my ski tails off the snow just with body weight. Then he had me adjust so that I could do the same thing but just with my lower body, without bending forward at the waist. (This is the part I'm not sure I'm communicating correctly, or remembering exactly correctly.) Then he told me to do that at the bottom of every turn.
Sure enough, it got me to flex my ankles, and of course the knee bone is connected to the ankle bone, as the song (sort of) goes, and everything just clicked. In my case, I immediately connected to that feeling and knew that it was correct, which made it easier for me to keep doing the move once I'd gotten that kinesthetic memory back.