The way it works is fairly straightforward. Your grinder actually slots into this device. You take the grinder's existing cord and tuck it into the grinder as much as you can. The remaining powercord fits into a recess in the back of the Esatto attachment. The grinder's powercord plugs into the Esatto itself. One screw tightens up everything, and after this point, you have a weight-based dosing grinder, with guaranteed accuracy better than 0.5g, and in reality, accuracy down to 0.2g or tighter (all numbers subject to final testing, which is happening in May). For all brewing methods save for espresso, 0.5 grams accurate is not only good, but I'd go so far as to call it near perfect: I would challenge anyone to tell any difference in a cup of coffee brewed with 28g vs 28.5g. It cannot be done.
For espresso, you want more accuracy, and 0.2g is not only within acceptable range, I'd say it is definitely within a target range that even the world's top espresso judges would be hard pressed to taste a difference in. In coffee cuppings, where the average measurement is 12-14 grams, accuracy is also extremely important but again, a 0.2 gram variance is within most cupper's tolerable limits.