Complete all events to unlock everything in Free Roam mode. Select a vehicle, then turn slightly to the right to find the two large walls that are used for wall drifts. Drive between them to find a hole in the ground. Enter it and drive through the underground passage to reach the speeddome Slowly reverse into a boost pad and keep [Reverse] held. Your vehicle will move back and forward a few times before stopping on the edge. Keep [Reverse] held until the trophy is earned.