When a wing in the form of an aerofoil moves in air, the flow of air over the top travels faster and creates a region of low pressure. The flow of air below the wing is slower resulting in a region of higher pressure.
The difference between the pressures at the top and underside of the wing causes a net upward force, called lift, which helps the plane to take-off.
When the plunger is pushed in, the air flows at a high velocity through a nozzle.
The flow of air at high velocity creates a region of low pressure above the metal tube. The higher pressure of the atmospheric air acts on the surface of the liquid insecticide causing it to rise up the metal tube.
The insecticide leaves the top of the metal tube through the nozzle as a fine spray.