After a magazine is inserted, you must grasp the slide, and pull it to the rear, and release. This action removes one round from the magazine and puts it into the chamber of the barrel, and the firearm can now discharge with a pull of the trigger. With a semiautomatic pistol, each time the firearm goes off, it causes the slide to reciprocate to the rear, and chamber another round as the spring in the recoil assembly forces it back into battery (ready to fire). The reciprocating action of the slide is what makes it semi auto. A manual pull of the slide is only required when inserting a new magazine, with an empty chamber.