I think it’s a different thing actually. When you’re forcing the water in, you’ve got a pipe with pressure greater than the water. Pressure is a measurement of

I think it’s a different thing actually. When you’re forcing the water in, you’ve got a pipe with pressure greater than the water. Pressure is a measurement of force per unit area. The pressure of the water presses evenly on the tube walls, but there is no tube walls at the opening. So, there is a spot where there is no pressure on the tube. This means a net force because that lack of pushing on the opening is not canceled out. Since the pressure differential is in the direction of the rotational axis you get rotation. In the reverse, there is also more pressure in the tubes than in the water. However, the spot where there is a net difference in pressure is the drain. The drain has an opening putting less pressure, so we get a net force. However the direction of the net force is perpendicular to the rotational axis—so we get no rotation.