There has been some discussion about use of diodes, so I thought I'd start a separate thread for it.

The common use of a diode is as a one way valve, letting electricity pass through in one direction and not the other.  For the 12 volt electricity in your car, the alternator is a small generator making AC power, and diodes are used to allow the juice out when it's in the positive cycle, and block it when its in the negative cycle, thus giving out DC power to the car.  In this case the diode's small consumption of 0.2v to turn on is ignorable.

In a git, however, the pickups only generate about 0.3v.  Now you can't ignore it, but you can make use of it.

If you put a diode between the tip and ground on your git's output jack so that it acts as a short to ground, you can get a sort of overdrive/fuzz/clipping effect.  the pickup spits out a nice wave and when it reaches the diode-acting-as-a-short, the diode will turn on only when the wave gets above the diode's turn-on voltage.  the beginning part of the wave is too weak and the diode ignores it and it passes on by to the output jack.  when the wave is about to reach it's peak the diode turns on and keeps a steady 0.2 v drop across itself and the excess energy shoots through to ground.

so instead of a nice smooth wave going down the cable, it is "clipped", the nice smooth crest is cut flat giving a distortion to the sound.

This is a little bit simplified, actual voltage produced by a pickup (mag/piezo) varies depending on type, and the cut-on voltage of a diode also varies based on type.  a weak pickup may never turn on a strong diode and no clipping happens, and a strong pickup with a weak diode could turn on too soon and clip far too much of the signal.