Press the gas a little and start going down the ramp but hold in L2 for five second or less then let go hold the gas after that, then hold and let go of R2, but go of the side/top of the ramp and u should get it

From your starting point on the dock, back up a bit and look to your left. Ride over to the set of jumps that are at the other end of the barges. Once in the area, ride over the jumps to your left until you come to the jump that is directly left of a palm tree. Past this jump are two landing slopes: one that is directly in front of the jump next to the palm tree, and one that is behind it and to the left. Ride over to the second slope, and face the right hand corner/side area of it. Continuously ride into this area, trying to push your way through it. With some speed and the proper placement, you should be able to ride right through the corner of the slope and ''fall'' through the ground, eventually re-spawning back onto the dock where you started out. During this fall you will also be able to string together as many tricks as you wish for an insanely huge multiplier without the fear of crashing, because when you re-spawn back onto the dock, the game still acts as if you're in the air (but be careful with the gas and brake/reverse, however, because the game also acts as though you're on the ground as well). When you're done with your multiplier, speed off into the level as you would normally. If done correctly, when your jump distance meter appears, it should read more than six hundred feet.

Enter JIH345 as a cheat in the options menu.

Complete Freestyle competitions.
Complete Master Supercross.
Complete Career Mode to unlock.