It is the principle of a connected vessel. The pressure on a liquid in a closed vessel is equal everywhere. The siphon is filled with water and there is no gas. The water level at the end of the siphon is high, and the water outlet is closed with a palm or other object. The pressure inside the tube is equal everywhere. After everything is set up, open the water outlet. Although the atmospheric pressures on both sides are equal, the water level at the water end is high, and the pressure is strong, which drives the flow out of the outlet.