From the Super Mario Wiki
Ski Cross is an event in Mario & Sonic at the Olympic Winter Games. This event is similar to Snowboard Cross except the players must ski down the slope as fast as they can to reach the finish line.
In the beginning of the race, the player can flick the Wii Remote and Nunchuck down to Start. When the players reaches a jump platfrom, they can swing the remote and Nunchuck up to jump for a speed boost and also do tricks by moving the wii remote left, right, up or down. The players can gain extra speed by flicking the remote and Nunchuck forward and also speed up without jumping off hills.