Like was said before, Squats & dead lifts...PLUS Calf exercises! Also throw in a lot of core work (abs/low back etc).
If you are mechanically inclined, build a box in your back yard and some sort of ramp to ride down to hit the box. You can use an old tarp and a lot of soap to make the tarp slippery. Once it's slippery and you have a ramp to ride down to get some speed, just practice hitting your home-made box all spring/summer long.
What am i supposed to ride to his the box?
Thanks alot for all your help guys! im really taking all this information in... Explosiveness makes sense now.. from what im understanding its haveing the endurance to be able to ride from the top of the mountain down to the terrain park and still being able to hit those big jumps... I was planning on doing alot of lower abdominal dead lifts and lower back lifts.... I was also thinking for calves to run on a incline? I wanna build endurance in my legs so i thought maybe that might help with swimming to complement it