If the vaulter is able to hold a push up backwards on the ground & with increased legs, try this exercise on the stability ball. It helps you to hold your feet on the croup during the technical tests exercise.
Put your feet on the ball: be very elongated, don’t let your bottom fall off. Lift your leg until it is 90° to your body. Alternate the legs. You also could hold it for a few moments at the highest position.