There's another earlier type dump valve that lowers the line pressure when the trans is in top gear. In top gear, the two clutches are applied and the theory holds that it takes more pressure to apply a clutch than it does to keep it applied. IOW, once the clutches are applied, the line pressure can be reduced without decreasing the grip of the clutches.

Some racers would plumb in a ball valve or solenoid valve to bleed off some of the line pressure; this reduces the parasitic loss from the transmission pump.