to do the boost, when the 1ST crystal turns red, hold down L and R. When the second one is red, hold UP on the D-pad. Lastly, Press A when it the race starts. Then you get a nice lead over the other racers with that.
first go to the volcano,then speed fast[i say using a boost]. Then you should be over the impassable fence. Then just jump off[but make sure you are not just driving off]then ta da you fell in lava. You just get ballooned back to the start then