Most of us who started riding in traditional "English" lessons (Nancy...?) probably remember learning to post with no stirrups at all!
Oh yes. And jumping little jumps, no stirrups, no reins. But those old exercise do help a rider develop a seat independent of hands and feet. And they're not specific to English riding either. I dropped a stirrup in a dicey situation on Hunter one day out on a highly technical trail and still managed to stay in the middle of him without grabbing his face. Of course in my mid-sixties and overweight, I do not ride like I did at 30 or 35, but the more those skills are honed, the more automatic they become and the safer we are when riding. Nancy