When the differential unloads all the power from the motor or engine on one wheel. This usually occurs during a sharp turn with an off-road vehicle that has a soft suspension. The result of the soft suspension is the entire vehicle leaning (body roll) to the left or right, and lifting one of the tires up off the ground. Cars and trucks with locked or spool differentials do not experience wheel spin. Rock crawlers must have a locked or spool differential.