When your slide locks back on an empty magazine it's time for an emergency reload. You are in the middle of a gunfight, and you feel a dead or weird trigger. Look and move. When you see you are out of ammunition you reach down with your support hand, and you grab and extra magazine. Simultaneously, as you are bringing your fresh magazine up with your support hand, you are releasing your spent magazine with your firing side thumb. Ideally the two magazines will pass in midair. Once you have the fresh magazine in the gun, you send the slide forward, either by reaching across with your support hand and then pulling the slide back, or by using your firing side thumb to depress the slide lock. You then point back in on target with your freshly loaded firearm. This is an important technique to ensure that when you are in a gun fight, you are back in the fight as soon as possible.