The idea of a racing tuck is to make yourself as small and aerodynamic as possible to minimise air resistance and get as much speed out of the hill and your board as you can. There are quite a few variations on a good tuck, but here?s a pretty standard version to get you started.
From a normal riding position, pivot your back foot so that your toes are facing forwards down the board. Bend your knees and sink your weight down so that your back knee tucks in behind your front knee. Keep your weight fully over the front of the board ? this will make it more stable. Try and get your chest as close to your front knee as you can, then tuck your arms in behind you. Keep your hands wrapped around behind you out of the way. To avoid straining your neck too much, keep your head down and look up with your eyes. Stay relaxed to avoid the wobbles.
The Next Step: you can always go faster. Take a deep breath and hold that tuck as long as you dare!
