When a jam occurs it, if the gun is so equipped, it may trigger the lock device as a safety measure. By locking it open the next bullet in the chamber will not advance when you clear the jammed spent casing, preventing you from shooting someone while attemting to clearing the chamber. This tends to freak a novice out as they are unsure about clearing the chamber & even if successful may not know the lock has been activated preventing them from firing a second bullet.