For all manual transmission models except Previa, and the 1988 and newer Celica, there is a small button inside the vehicle labelled Center Diff. Lock and when activated, the transfer case locks, resulting in the front and rear differential input shafts being driven at the same speed. The result of this is that if the vehicle is in a situation where traction is uneven or poor (i.e. muddy or snowy), the car can move with traction in either the front or rear wheels.