I saw a kid a few weeks ago riding a gas powered surfboard down the river near me. Real weird to hear and see a weed whacker engine on the back of a surfboard on a river in Illinois of all places.
As you can see, there are handlebars which you can pull up at the same time you apply the brakes, that way the weight shifts back against the force of the brakes. You have to have your hands on the handlebars anyway to operate the lever.
So? The moment you push on the pedal the wheel gets faster and the back of your head would meet the ground if you wouldn't counter it with your own weight. The same principle applies to braking.