Years ago we used to make links on computer boards with a pencil for temporary configuration changes, so I thought I'd try it on a Flashlight.
It still works!
Rub the pencil back and forth until you can see a graphite track and if you don't like that selection, just rub it out and try again.
You know, I thought of that a while back but didn’t try it.
That’s an old rotten trick we used to pull by leaving a pencil trace around the inside of a distributor cap….how to immobilize a vehicle in one easy lesson. Of course distributor caps have gone the way of the horse and buggy.
The old Trumpys had points cover held on by a spring - easy to remove and replace and a cigarette paper between the points would really wear out the victim trying to kick start it.
Hey! most of my neighbours have horses!
The AVR chip used on most drivers has 40K internal pullup resistors. To get a reliable “0” into the processor you need under 10K of resistance to ground. Kinda iffy for a pencil trace on a hard circuit board where you need to “color” the vertical side of the etch where it steps down to meet the circuit board material. On paper I needed a 1/8” wide pencil track to get under 10K ohms. Probably OK for testing, but don’t count on it in the long run…
ah the good old overclocking technique 
But most CPU chips have much larger valued pullup resistors in them… the resistance of the pencil mark is much less important…
Older CPUs had bridges that needed to be connected in order to set the clock multiplier and FSB.
yep! here’s an old article i wrote
http://www.overclockers.com/blowing-your-bridges/
blow and pencil to get your desired OC
LOL... I had the Goldfinger device for my old SlotA Athlon CPU. I somehow accidentally fried my CPU though and it didn't work after that... 
What about the pencil trick for Radeon 8500 voltage adjustments? And I think 9700 series had pencil voltage mods as well……
I do remember Athlon Tbird multiplier mods. And unlocking the multiplier with pencil mods on Tbred A and B CPU’s……