What is a spring powered air rifle?

A spring powered system uses a spring that is inside an air tube. The spring is attached to a piston. When the rifle is cocked the spring and piston are drawn back and locked at one end of the air tube. The other end of the air tube has a small hole that is directly behind the pellet and rifle barrel.

When the trigger is pulled the spring is released and it pushes the piston down the air tube forcing all the air through the hole, and that fires the pellet down the barrel.