Ok, I knew.
The two windings on a humbucker have to be in opposite polarity magnetic fields to work. That's why each bobbin has its own set of pole pieces. One bobbin has north facing up and the other bobbin has south facing up. With opposite polarity on the magnets and opposite polarity on the windings, the signals from the two bobbins are in-phase when the strings are moving thru the pickup's magnetic field. External magnetic fields, like the 60Hz field coming from an amp's power transformer, produce out-of-phase signals at the two bobbins and those hum signals cancel.