Quick explanation:
You set up your line at the start of the draw in the normal way. Shift attention to the clicker, draw until you can see that it's ready to go. Shift focus back to the target (you should still be pointing at the middle, otherwise you've done something wrong...), complete the draw. With increasing levels of control it's reckoned that you can cut down the "last little bit" to sub 1mm.
Advantages: you can see when you've done it wrong (no hanging around at full draw), you can use the visual feedback to help do it right...
Disadvantages: you need to learn control, but then you need to do that anyway
It's been said that using this sort of technique, you can start people out on a clicker, almost immediately - cut out the "oh you've got to wait a year or so for your draw length to stabilise" rubbish.
Don't get me wrong, it's not a quick fix. Just probably a better technique than "pull and hope it goes click". Especially for intermediate level archers. The top level ones probably have such a consistent draw length, they could do without a clicker and get reasonable results!