The gear changes must be made with a mechanical linkage system, so paddle-shifters were outlawed. However the system was re-allowed in 2015. There was no center differential (earlier it used to be 3 differentials, with a center/3rd differential included), but the new regulation allows the only front and rear axle differential and a mechanical clutch to disconnect the rear axle during handbrake use (to reduce cost and make the cars' driving style more exciting again for both spectators and TV broadcasts). These two differentials must be mechanical, without electronic control or hydraulic or viscous systems (from 2006 to 2010 the center differential and previously all three could be active).