Legendary Diving Coach HOBIE BILLINGSLEY was often heard to tell a diver "Don't let the water dictate your direction upon entry." In a nutshell, this means that a diver should maintain a tight straight-line body position throughout the completion of the entry. The dive is NOT over until your toenails disappear underneath the water.
If a diver does not maintain a tight body when they enter the water, the water might force them to move in a certain direction that makes the dive appear to go short, over or maybe look twisted in the eyes of the judges -- all things that reduce the scores given to the diver.
Divers must learn this most important of skills from the very beginning of their diving careers. A good drill is to have them lie on their back with legs straight, feet together, toes pointed and a "flathand" grabbed and then on your command "STRETCH" -- they try to make their body 10 feet long and hold this tight stretched position for 10 seconds. Relax and then repeat 10 times. In addition, every practice should include time spent on learning to enter the water straight (read: VERTICAL) with this tight body position and good form (Legs Straight, Feet Together, Toes Pointed).