somewhat of a newb question...double set triggers
So I was ready through the 7X57 thread and saw a picture of a rifle with double set triggers. I never really knew what they where (of course, I assumed they were really nice, and expensive) but I kinda figured it was a description of the internal workings. The rifle pictured was single barrel and double triggered...Soooooo that makes me have to ask: What exactly are double set triggers, how they work, what the advantage is, and is there any downfall to them other than price?