You use your thighs posting , if your not sure what your using to post drop your stirrups and if you have zero balance you have been using your stirrups or your knees. Both have risk of injury when you use your knees you overthrow your center of gravity and when your using your stirrups to post your not able to maintain your seat.
You can post bareback. It isn't about pushing yourself up and down, but rather about moving with the horse, making the trot easier on both you and your horse's back.
You should have a deep heel but make sure you're using your inner thighs and post with your hips moving up and forward. If you just stand up in your stirrups when posting, you won't develop the correct muscles that you will need later on, especially if you're jumping or even just cantering. A way to help develop a good post and strength that inner thigh muscle is to post trot without your stirrups, if you don't feel comfortable that your horse is going to react the the banging of the stirrup, have someone put you on a lounge line and you pull your stirrups up and have a controlled circle! Hope this helps!
i'm not sure i get the question but when you post you just put all your weight in your heels and just stand up almost all the way every other beat