In stock form, the apply fluid for the 1-2 shift is routed through a small orifice (the B orifice in the pic below) but the return is through a larger orifice as the check ball falls away. Removing the ball allows the fluid to flow through both the small and large orifice; this makes the shift much firmer. For a less firm shift, the ball remains and the size of the small orifice is increased.