Understandly so, double set triggers are not very common these days. Here's how it works. The double set trigger is not to be confused with a two stage trigger or double triggers on double guns where one trigger operates each barrel. With double set triggers, there are actually two triggers. The front trigger is the only one to release the sear. Unset, ideally it would be 3-4lbs and used for most purposes. Squeezing the rear trigger "sets" the front trigger. In this mode the front trigger typically breaks very light, ideally under one pound. The idea is that you will use it unset most the time and only "set" the front trigger for those times that a lighter letoff will be advantageous, either on the range or in the field.
That early "bucket scene" in Quigley Down Under is a good example of a double set trigger in use.