First the rider must learn where the leg should be for balance (used to be called balance position) to keep from falling forward or backward. Stand up into 2 point, put leg too far back (from knee to heel) and the rider will fall forward. Then put leg forward and the rider will end up falling backward. Then learn to do it in walk, trot, canter. Dropping the chest too much (butt out behind) will also cause the leg to slip back/rider to fall forward.
Intially the rider will tend to turn the toes out, grip with spine of boot. So, turn the toes more parallel, this created a better way to keep balance w/o gripping and STRETCHING into the heels.