HERE we can show gymnasts examples of leaving the floor or apparatus fully stretched.
YES, you only need to show that stretch until leaving the floor / apparatus — but a little bit of exaggeration could help the gymnast understand more quickly.
BEST of all are the illustrations by James H. Stephenson.
Neither book was focused on biomechanics. Most groundbreaking was his taxonomy of how to define good technique. Any future computerized analysis should use these principles.