Concertina plungers work by a build-up of water pressure, which is drawn into the body of the plunger by the bellows. When plunging the plunger, at least half the cup should be submerged in water.
When the plunger is placed into the water at an angle, the water replaces any trapped air, forcing it out of the cup.
When the handle is pressed down into the body, it forces the water in the cup of the plunger down the drain, and against the blockage.
As the handle is pulled back, it creates a space, which the drawn water promptly fills.
As the water is drawn up into the bellows, the water movement created in the pipe helps to break up the blockage.
The handle is then forced back down, compressing the bellows, forcing the water back down the drain, pushing and flushing the blockage away.