Well, this is the Tilting Barrel.

Since before 1900, J. Browning was trying to create a mechanism, that would allow the breech of the gun to remain closed, during the moments when the pressure from the expanding gazes produced by the burning powder would be high (that is until the bullet exits from the muzzle of the barrel), but would allow the breech to open up, after the pressure drops, in order to feed a new round in the chamber.

Browning initially thought of the tilting barrel. In simple words, the solution Browning improvised did three tasks :

1. The breech bolt of the gun, was transformed into a "slide" that encompassed the barrel.

2. The pistol's barrel was locked to the slide, by means of matching ribs cut in the barrel and the slide, as shown below.

3. Two toggle links, allowed the barrel to drop free of the locking ribs, as the barrel and the slide moved rearwards, opening in this way the breech.

This two-link system was first used in the Colt Model 1900 pistol. It was improved later on, by eliminating the front link, and replacing it with the Barrel Bushing, in the front of the slide. That is the design that M-1911 still uses today, even though Browning still impoved on it, when he designed the Browning Hi-Power P-35, where even the bushing was eliminated, in favor of an angled opening in the front of the slide, thru which the barrel passes.

This ingenious design of Browning, used the simple barrel link, which was attached to the slide stop shaft, to tilt the barrel up and down. Later, again when designing the Fabrique National Hi-Power P-35, Browning improved on the tilting mechanism, by eliminating the barrel link and replacing it with a kidney-shaped opening, machined at the bottom of the rear part of the barrel, as shown below.

Also, more modern engineers, dispensed the locking ribs on top of the barrel, and replaced them with a simple recess in the front, upper part of the chamber (such a system is used in contemporary Glocks and Sig Sauers).

Now that you understood this simple basic principle, you should go back(click your right mouse button and select "Back in Frame") and read how it works during the firing cycle of the gun.