I have fast reflexes but when I dive into the water my body resembles a starfish more so than a tight streamline. (That is me to the right.)
Bonnie Adair, the head coach of LMU Women's swimming, had me do a start drill so as to improve my streamline. She had me dive into the water with a pull buoy between my legs and the object was not to let it fall out. If you do it right you will maintain a tight streamline with your legs, pressed together and hydrodynamic. If you do it wrong, and you are a man... Well; I will phrase it this way, it could bump into your $1.50 if it flies out. Ultimately you are rewarded if you streamline and unrewarded if you are not.