The spotter counter balances the gymnast so they can “feel” the correct position before initiating the jump for backward handspring.
The RED line is the goal. Thee figures on the right (A,B,C), common errors:
For fun, with confident, experienced tumblers on a trampoline — the coach can release the grip, asking the gymnast to do an almost instantaneous back handspring.
This diagram comes from an excellent longer article by Steve Bonham, from Georgia Southern University: Body Awareness Drills
Gymnastics Coaching.com 