Is it a boost? is it an overdrive? It's a treble booster! Originally designed to boost the highs this wonderful "boost", and I use that term loosely ,not only pushes the front of the amp harder resulting in more drive but it especially boosts the treble making it the perfect pairing for a very thick and fat sounding profile.
With humbuckers you can pair a treble booster and a darker amp and you get a very special classic rock tone. Check it out!