Knee
I keep my left foot down on the backswing too. I think some will say starting with the feet is the way to go. For me it feels like I start with moving my left hip to the left but realistically, my weight is probably moving left prior to that feeling. I would guess my knees are moving left first.
I think keeping your left foot down is great (recommended by Joe Dante in his books). For me it increases my stability (reduces sway).
For me one problem with GOLF has been what is really happening versus what it feels like is happening. Your left knees moving left is probably a precursor to left hip moving left too. Move from the ground up (sounds fine to me).
I know Brian will have some thoughts on this which will be far more profound than mine.......