ok this will make your board invisible for the combo you do it in.
Firstly, start the Tony Hawk 2000 Highscore game.
Then, just go anywer moderatly high and nail the trick. While at the top of the NTT, force crash yourself and press start.
Then hit restart goal and while the loading screen is up keep trying to get into manual so when its done loading your skater will fall, hit the ground, and then go into manual with an invisbile board.