I’m not sure what @eightpondfarm is seeing. But, I can say that I’m seeing absolutely normal posting in both full speed and 1/4 speed. If I watch what the horse’s back is doing, the rider is 100% in sync. If I get wrapped up in watching the outside front leg I could convince myself that there is something wonky, but then I remind myself that the motion of “following” that front leg is not exactly as advertised by instructors to beginner students. We need to be starting our way back down from the peak of our post as the horse touches the ground. If we wait until the horse is pivoting its weight over that leg, we are going to get behind the motion and smack the saddle as the horse’s back is already pushing back up again.
I think we also need to remember that the initial touch of our delicate ass to the saddle does not indicate full weight on the horse’s back. That takes a moment as our butt muscles/flab compress. The reverse is also true, our initial lightening of our seat happens before it is visible that we are indeed rising.
Just what I am seeing, my thoughts. I’d also like to know what EPF is seeing because my eyeballs are rusty.