First, start a practice race. When the number 3 begins to fade, press and hold down X. When number 2 starts to fade, let go of X. When 1 starts to fade, press and hold X. If this was done done correctly, you should immediately start off with a extra-fast start and a 4+ second speed boost!
Unlocking hard races
To unlock the hard difficulty races come in 2nd or 1st in all three middle races.
Unlocking MIddle Races
To unlock the middle difficulty races come it 3rd, 2nd, or 1st in all three easy races.