There is a piano hinge all the way across the back of the holster, it is spring loaded. The button (green arrow) is inside the trigger guard. When you have a shooting grasp on the gun, you stick your finger in the trigger guard and push the button. A lever trips the catch (red arrow) allowing the holster to fly open. There is a steel "hook" (blue arrow)over the trigger guard to prevent drawing without opening the holster.
The entire holster is leather covered steel. Its very heavy. There is no "draw". You hold the gun in a firing position, push the button and the gun is free. Any angle, any position. Its a blindingly fast draw.
All the old revolver holsters had exposed trigger guards. The Jordan River holsters, the Threepersons. Guns were dangerous, shooting had inherent risks. Lawyers, and duty guns with short 5.5 pound triggers were not the norm. Most AD's occured during reholstering, not drawing. It was thought a covered trigger guard would be more dangerous. If your finger is on the trigger, and, the trigger guard is covered, when the gun goes in, the finger stops, the gun keeps going and the trigger goes back, bang.
The next generation was the Bianchi Model 27 (readily available, it was a a take off of the Berns Martin holster that was awesome, but, never that popular). It was a true clamshell holster (with a covered trigger guard) You had to push the gun through the front and, it popped closed when the gun came out. Since you were actually ripping the gun out, I think it would NOT be a good idea to have your finger on the trigger!
For your entertainment:
http://www.youtube.com/watch?v=ViX9D9TJ8k8&feature=relmfu