Randy's explanation is good for double set, but doesn't answer the question about single sets. Personally I don't like single sets because it is difficult to set the trigger with the gun in firing position. If you set it in any other than firing position you will be handling an arm with a very light trigger and have a very real possibility of an AD. That being said, I installed one of the single sets from Susi at MBS. Her explanation was to build a jig to hold both the trigger and the lock so that you could adjust fore and aft to find the optimum positioning. It is a touchy situation because if the trigger is too far back you will have a good pull unset, but you may not have enough power in the set mode to trip the lock. if you get the trigger far enough forward you will have the power you need, but you may not have a very good unset pull. I ended up installing mine so that it had a good unset pull and never use the set function at all, so I now have an expensive single trigger, but it has a good pull.