This plugin works by resetting the ball and car to the opposite side of the field whenever one of them gets too close to the ceiling/floor/walls, creating an infinite air space to practice air dribbling. The plugin also auto-resets the ball to be above your car if it gets too far away or if it is beneath your car.
Look in the 'Plugins' tab in bakkesmod and find 'AirDribbleTrainer' from the list of plugins on the right for directions on how to use it.