You shouldn't feel like your ankle is going to snap when your in half seat or your position isn't correct. I haven't had this problem but maybe its because you put ALL your body weight down to your heels. Try using more of your core and thigh muscles to help lift yourself up. When I first started practicing half seat, I need to really use my thighs to lift myself up. Try doing that. Also, have a friend or trainer correct you and see how you ride. Maybe have someone videotape how you ride as well. I hope this helped!