Using a prop like a medicine ball as a depth gauge when teaching the squat can be helpful by giving the client feedback before he or she develops better body awareness. This should be a temporary solution, however—everyone must be weened away and learn to recognize proper positioning and depth without external help. Also avoid allowing your clients to use the prop in question to reduce the demand of the movement, e.g. bouncing off the ball.
Pictured is Eva T
performing Tabata squats during a Ski Fitness seminar at NorCal Strength & Conditioning