Once you "pull" the trigger in, it is constant on. The dial on the trigger (it's kind of hard to see, but notice the round piece on the trigger), allows you to adjust the speed (RPM). This is a low RPM drill so you can really dial in the speed to whatever RPM you want and get the crush you want as well.
This isn't a great picture but you can get the idea. This was my first crush using this setup. I got 80% eff. out of this.