In series the coils of a humbucker combine to give you more signal and resistance (this is the normal sound), so for example a 9k pickup will have 2 coils of 4.5k. If you wire these in parallel, they will drop the resistance to 2.25k and the sound will get brighter but a lot of the midrange will be cut (liek when you have the in-between settings on a strat).
