A phase inversion is exactly that. We flip the phase of one pickup to achieve a cancellation effect.  Basically what that means is the + and - wires are switched on 1 pickup only. In the case of the Jimmy Page harness, it's the bridge pickup.  Every guitar pickup produces a waveform that can be viewed on an oscilloscope, and they look very much like waves in the ocean.  It is a matter of physics that any two waves equal and opposite will cancel each other out and equal "zero" again, just like...