it works like a venturi, similar to a chemical intake on your pressure washer, your pressure water puts water through it and the venturi part sucks the water up, I think it's runs at your litres per minute x 10, ie if you have a 15 litres per minute machine the water it can take up is 150 litres per minute - 15lpm x 10 = 150lpm
great bit of kit for draining blocked manholes etc