When the rifle is fired until the magazine is empty, the last shot (as every shot) pushes the "hammer" piston backwards. Normally, this would happen, and the piston would push a new cartridge into the chamber on it's way forth. But now, since the magazine is empty, it will get stuck in the rear position.
This allowing the rifleman to change his magazine, and press the release button, bringing forth a cartridge from the full magazine, and the weapon would be ready to fire. The button is located on the left side of the rifle, opposite the magazine drop button.
And I'm not talking about the forward assist.
Most newer automatic rifles work this way. HK-416, HK G36C, Colt C8 Carbine, and so on.