It replaces the fixed main jet and carb retainer bolt with a needle jet, and a jet needle. You're left with a knurled knob that sticks out the bottom of the carb and allows high-speed adjustment. It's great for dialling things in, but quite honestly, once you're dialled in, you should never have to touch it again. If your tear down the carb, you'd better have taken notes to find the right setting again!
On the other hand, if you plan on making many changes to your engine over time (and NOT replacing the carb, it's a lovely thing.