hold down the magazine spring so that you can dry fire without locking the slide back.

those parts are made to simulate bbs inside the mag, so you can dry fire the gun without having to hold down the slide catch...

really great if you use the guns for filming props... just not in public obviously.

Watch out tho, if you insert them improperly, they can either get stuck inside or screw the slide. Which can in turn screw a recording session. :(