extract form this great bending page :
These are simply metal contacts — drawer knobs, threaded brass light fixture balls, whatever — that are wired to the pair of circuit-bending points. Each of the two circuit points goes to its own body-contact. Nothing is wired between them at all… no switches, potentiometers, sensors… nothing. These contacts, when mounted on the instrument’s case, are meant to be bridged by the player’s body. This placing of human flesh amidst the circuitry, now conducting electricity as surely as any other component on the board, turns the body into a potentiometer of sorts. A variable human resistor (but then, mustn’t we all be already?).
Body-contact circuitry points are discovered in the exact same way as the circuit-bending pairs… with a test lead system. However, instead of the alligator clip test lead grasping a small jeweler’s screwdriver at each end, you do. You simply hold a screwdriver in each hand. The search process is the same as before. The circuit makes its usual sounds while you listen to the changes that might occur due to the electricity now flowing into one screwdriver, through you, and out the other screwdriver back into the circuit. If good points are discovered, they are wired, as mentioned before, each to a metallic body-contact mounted on the instrument’s case. These can then be touched by the player, creating the same body-circuit as discovered with a screwdriver in each hand.
Rarely is this electricity ever felt by the player. In a certain 9 volt amplifier, Reed’s first circuit-bent instrument, the body-contact system did deliver small shocks. But nothing like the static shocks of wintertime carpet-strolling, or worse, the dangerous shocks that befall most musicians now and again from improperly grounded stage equipment.
The important note here, however, mentioned before and worth repeating endlessly, is to try these circuit-bending techniques ONLY ON BATTERY-POWERED AUDIO DEVICES OPERATING ON AN ONBOARD BATTERY POWER SUPPLY OF 6 VOLTS OR LESS. Trying to circuit-bend anything plugged into the “house-current” of your AC wall outlet, directly or through an AC adapter (power supply, power converter, “wall wart”, etc.) is OUT OF THE QUESTION!!! NEVER TRY TO CIRCUIT-BEND ANYTHING PLUGGED INTO A WALL OUTLET.