If you look at the Just Right drawing, you will see that the key item is that the hot water pipes must always proceed upwards until the return line connection, and from there they must always proceed downwards. There is nothing magical about it. It is a gravity system and is ruled by the same laws of physics. Hot water will rise and cool water will drop. If there are any down turns on the hot water supply to avoid beams, etc, then the system will not work without a pump.