That is known as a siphon. It's the same way your toilet empties. The bigger the height difference between the water level and the discharge end of the hose the faster it drains, up to a certain point. You can start the siphon effect even without suction by slowly imersing one end of the hose into the pool and then keep feeding in the rest of the hose slowly until the other end is also under water, you have to make sure that you allowed all the air to get out of the hose. Put your thumb over one end making sure that the other end remains under water. Lift it over the side of the pool and get the hose end below the surface of the water before take your thumb off. The water should start to flow and will do so until either your pool water level is even with your discharge end in elevation or you drain so much out that it sucks air at the hose end in the pool. I use this method and weight the end down on a step so that it drains to that level and goes no further.