I agree with the above. It's a lighter driving seat. Just to touch down and remind your horse to move forward while using your seat as an aid rather than "motoring" the horse with your seat the entire time. Or constantly being in your half-seat (shallow 2-point), which can allow your horse get really flat at times.
Correct me if I'm wrong. But isn't the posting at the canter also used to get deeper into your heels? Especially when you are jumping around a course?
While Riding A Horse, We Borrow Freedom