Alright, so I just tried it on the balance ball, and to me, it feels good, although you dont have the impulsion like you would on a horse, you kind of have to imagine it yourself. But I can already feel it in my thighs lol.
If you sit squarely on the balance ball your legs are bent at the 90 degree angle you would on a horse, almost, and your feet are planted on the ground, so wouldn't that be ideal foot positioning for how they should be in the stirrups? Anyway, I started bouncing up and down, and on the 2 count, I would squeeze and raise myself, like I would imagine I would on the horse. To help keep my hands still, I stole my son's little wooden chair and put that in front of me to rest my hands on, not to pull myself up on. Possible good way to practice the post?