Passive Octave

Two coils connected in parallel-opposing (parallel and out-of-phase) can produce an octave up effect. Normally this is about the worst possible wiring you can do for a 4 conductor humbucker. Don't expect this to sound good outside of the weird octave effect.

A neck position humbucker is the best candidate for this trick.

Some humbuckers may work better than others.

The theory:

The voltage combining at the "hot" side are two signals out-of-phase. If the signals were exactly the same and opposite, there would be 100% cancellation. A typical humbucker will just sound weak and strange with this wiring due to partial cancellation of the signal.

If enough of both signals remain audible, the ear should detect some degree of octave overtone. This effect will be greatest where string rotation is most symmetrical, which is about the 12th fret. The perception of an octave comes from the two signals "filling in the gaps" so the wave actually appears to be rotating twice as fast as the actual string.

If the loss of volume from the cancellation can be tolerated, it really does sound like you've switched on a clean Octavia effect by the flick of a switch on the guitar.