Reply #3: It's the distribution of your body weight across the board

[nothingshocksmeanymore]

If you know someone who does Tai Chi ask them to show you the Tai Chi body balance posture..this same posture applies on a surf board.
Also if you start out on a long board, it's easier but other surfers will hate you for ripping up the wave.

Think of speed more as momentum such as the momentum one attains when balancing on a bike for the first time.

You don't have to be going fast just gaining enough momentum for the water to support your sense of balance.

on edit: as with a canoe, the back foot controls direction just as the rear paddles do