There is a steel cover that goes over the back of the trigger guard. (blue arrow) that keeps the gun in the holster.
There is a leather covered button (green arrow) that is inside the exposed trigger guard. When you depress the button, it is attached to a lever that runs along the back of the holster to the latch (red arrow) and moves it out of the way. There is a steel loop on the opposite side of the clamshell. When it releases, there are two coil springs in the back (now middle) of the two holster halves that force it to fly open.
The entire holster is steel, covered both inside and out, with leather.
I hope that makes sense.