In order for the plunger to be effective, it needs to have a tight seal between the edge of the item to be plunged and the sealing edge of the plunger.
A tight seal is gained by tilting the plunger when entering the water. This expels any air out of the plunger cup and ensures the cup is full of water.
Water cannot be compressed, but air can.
If there is air pressure under the cup, it can be compressed, resulting in air escaping and leaking out from under the sealing edge of the plunger. This would break the seal between the plunger and the blocked item, making any plunging efforts ineffective.
When a good seal is obtained, water is forced down onto the blockage as the plunger is pushed down by hand.
As water does not compress under pressure, a build-up of water pressure develops with every push of the plunger.
However, when the plunger is pulled upwards (back), the pressure on the water is reduced, so that the water is under low pressure.
The motion of plunging in an upward and downward movement, puts the water under high and low pressure at a consistent rate.
The changes in pressure push and pull at the blockage, breaking it up and away from the walls of the pipe. This action, with the aid of gravity, helps to unblock the pipe, allowing the water to drain away.
Suction plunger
A suction plunger is slightly different compared to a cup or flange plunger. This type of plunger does not have a cup shape to trap air when it is submerged in water.
Because of its flat head, it can be inserted into the toilet gulley without trapping air (therefore does not need to be inserted at an angle) and a seal is easily formed.
When suction plungers are plunged up and down in the toilet gulley, they force the water against the blockage with high and low pressure.
The pressure will force the blockage apart or down the drain, allowing the water to flow freely again.