It's the double/single action capabilities of the gun. Similar to the anti-stress of the P99, the P22 can be fired in either double or single action. Typically on a hammer operated handgun that functions in double action, pulling the trigger will cock and release the hammer, firing the gun. This is what makes it double action. If the hammer is manually cocked back via the thumb, the gun is now in single action, and a pull on the trigger will only be needed to release the hammer and fire the gun. I have never held or shot a P22, but I would guess pulling the trigger back slightly would cock the hammer, and releasing it at that position would leave the hammer cocked, thus leaving it in single action.