The two coils in a humbucker are wound the same. So you connect them out-of-phase. (Red/white together.) Any external AC noise impacts the two coils the same. Since they're out-of-phase, the noise cancels. But, since the magnet lays with its north pole against one set of pole-pieces, and its south pole against the other pole-pieces, the string generates an out-of-phase voltage in both coils, which adds together in-phase, since the coils are out.
Does that make sense? :laugh2:
Look at this pic of a Custom that I took apart. You can just barely make out that the white wires come out of the top of the pic, and the black wires from the bottom of the coil: