You must complete the Championship and have all twelve cars. Then go back and practice (choose the checkered flags) and on the board selection screen, right click continuously on the Mattel logo instead of selecting a board, then click the green light... more
1. Do 50 or more stunts on a track.
2. After the stunts and crashes are tallied, you will see a short fireworks show.
I believe it is possible to do this on any track. The easiest way to do this is to go backwards on the track.