It's the trip not the pull. The first stage is pulled to the wall and held then it only takes 2.5 lbs to break the shot. It's the finesse at the end that disturbs the sights less than a solid 4.5 lb pull.
No not really, it still takes 4.5 lb to break the shot; it's 2.5 lb more on the second stage, not 2.5 lb total. Like ChrisF said, it's just a way to trick your mind, but the physical effect isn't much different than just preloading a single stage trigger by the same amount.
