This information is useful because of how phase works in the context of guitar electronics. Inside every humbucker pickup, there are two coils wired in phase with each other resting above a magnet. This means that the one coil is resting on the magnets South pole and the other coil is resting on its North Pole. This orientation of the pickups causes the signals to blend together well resulting in a thicker tone. If the coils were not attached to each other like with a stratocaster the orientation of the pickups could accidently be switched causing the pickups to be out of phase with each other which sounds very thin. This happens because the signal from the one coil cancels out all matching sounds from the other when the pickups are out of phase. Using this method for checking the pickups polarity can prevent that.

Step 1) If you don't have a magnet with an indicator of its North pole then you can use this method to find the magnets North pole if you know which direction North is. Basically I just float a magnet in a cup of water. The magnet naturally will turn itself to align with the earth's North and South pole. I use my iphones compass app to determine which pole is which.

Step 2) Now I mark the North pole of the magnet by glueing some red paper to its North side. I then check the humbuckers secondary coil which attracts the South side of the magnet, meaning that coil is North side up. Checking the main coil confirms that it is South coil up because the magnets North side is attracted to it.

With this marked magnet, you can tell the magnetic orientation of a guitar pickups coils. This can be useful when double checking the wiring of a guitar to check if the pickups are oriented correctly. You can even check vintage guitars to see if they have been been electronically altered if you know the orientation the manufacturer used for that model guitar.