the varisteam is an electronic speed controller for the pump motor.
the numpty on the front controls the speed the motor runs... and on off switch.
the varistream also has pressure sensors. so when you want to stop the water ie going between window to window, you have a flow valve on your hose line near the pole, or on your belt then when you close the valve- the varistream senses it and turns the motor off, so you dont waste water or electricity- much better than walking to your vehicle and back each time you want to turn off and on - and you dont waste water inbetween. rather than a full power torrent of water all the time, you can slow the pump and slow the flow.