For anyone with poor grip strength, racking the slide on a semi-automatic pistol isn’t easy. While you can strengthen your grip, it may not always be an option for everyone.
As written and reported by The Well Armed Woman:
A semi-automatic pistol uses the energy generated by the firing of the first round and uses it to expel the spent casings and draw in the rest of the subsequent rounds from the magazine. The first round must be chambered manually to get the whole process in motion in a semi-automatic. To accomplish this, the first round held in the magazine, which holds the round and is inserted into the grip of a semi-automatic, must be “pulled” from the magazine and aligned with the barrel in the chamber.
Take a look at the video below which shows the action of a semi-automatic. (it is in slow motion so you can see the action, it is much faster in live shooting) You will notice that the slide moves to the very back of the frame and then slides forward and draws the next round into the chamber. As stated above, the energy that is expelled in the firing of a round is used to perform this action. Obviously, a “first” round has to be fired to set this all in motion.
This is why we manually “rack the slide”, to get the first round into the chamber. Once the sequence is in motion, only a pull of the trigger is required to fire the firearm. Most semi-automatics will lock its slide open, automatically after all rounds have been fired from the magazine.
Once a new loaded magazine is inserted, the slide only needs to be fully released to chamber the first round from the new magazine.