Quite a simple way to explain how to make a competitive start.
I tried few times to dive from the deck, but I felt the deck might not be high enough for one to dive this way, the competitive start (or slingshot dive). While doing the tight streamline in the air maintaining almost horizontal to the water surface, if dived from the deck, there is not sufficient clearance below allowing one to enter the water in a nice position but rather with either belly flop or most likely "thighs" flop. So, it's advisable to practice this technique at the pool with diving block or stand.
It's not easy to find a pool that allows one to practice diving here in Taipei, nor in Tokyo (one can dive at Toshimaen). Diving is prohibited in the most pools, besides there is no starting block installed at the pool side, either. Before you find a pool that you can practice, why not try "mind exercise", watching videos several times and visualizing yourself diving from the block? If you are lucky enough to have a pool to practice, this "mind exercise" also helps a lot!
By the way, in the video, the male swimmer performed better dive than the female one.