The objective of carrying out this dive is to avoid having the wave drag you backwards. There is not an exact moment to dive, it is simply used so that you canget to the break point easier.
The first step is to press the board down, placing the whole weight of your body on the front part of the board.
Then you put your weight on your foot or your knee in the back part of the board, which will push the board toward the surface.
When you reach the surface you will be in the back part of the wave.
Finally, you should give a few more strokes in order to move away from the wave, so that you can wait for a good set.