This is a minigame where you ride a skateboard down a giant ramp to the end of the course. The level is a Score Challenge, so it's performed under race conditions. The faster you ride down to the finish, the more points you'll earn.
You have to start pushing/pulling the board to get it rolling, and then jump on for a super fast ride! You can grip anywhere on the board, and you can move faster by grabbing the nose of the board and repeatedly tapping the button.
"Acing" this level counts towards the "Play" trophy (clear every level without dying, save for those which end when you die.)