OFF-BOARD WIRING: Voodoo Labs Bypass System

I'm sure others have done it; however, this is commonly known as the Voodoo Labs bypass.  It's an interesting way of bypassing a pedal using a DPDT switch while including an LED indicator.  

Before 3PDT switches became available, designers had to choose between not true bypass with a LED or true-bypass without an LED (until the millennium bypass appeared, but that's another story).

Here's how it works:

PEDAL ON

The LED in the photo FET turns on, and the resistance of the FET drops down to a few hundred ohms - signal flows into the effect circuit, and the effect circuit output is routed to the output jack.

PEDAL OFF

The LED in the photo FET is off - the resistance of the FET increases to a few hundred megohms, blocking the signal from reaching the circuit.  The FET is also grounded.  So the signal sees a very large resistor to ground instead of a circuit.  The signal from the input jack is routed to the output jack. 

Or an even better explanation by R.G. Keen can be found here   

VOODOO LABS BYPASS SYSTEM

H11F3 PHOTO FET OPTOCOUPLER