Oh, and I wanted to add that when I am explaining to someone how to post for the first time, I always emphasise that they shouldn't think about it as 'standing in the stirrups' but as swinging their hips forward and upwards, as this is where the movement comes from and is the body part they should be focusing their thoughts on. As they progress you can refine the instruction, but I find this way of looking at it is particularly helpful for people who need to first understand the movement of posting.