When a piston-style plunger is used to compress a gas, for example in a syringe, the pressure on the gas is increased. Before the plunger is depressed the gas in the cylinder is at the same pressure as the atmosphere. According to Boyle's law, when the pressure on a gas increases the volume of the gas decreases by the same factor. If the pressure on the gas decreases then the volume of the gas increases by the same factor:
`P_1V_1 = P_2V_2`
Where P1 and V1 are the initial pressure and volume of the gas and P2 and V2 are the changed volume and pressure.
When the plunger is released the pressure on the gas decreases, so the volume will increase accordingly. For example, if you have 50 ml of gas in a syringe and you compress it to 25 ml by pushing the plunger, you will have doubled the pressure on the gas to twice that of the atmosphere. When you let go of the plunger the pressure goes back to atmospheric pressure, decreasing to one half. The volume doubles, returning to the original volume, as the pressure equalizes with the atmosphere.