OC-Melbourne has a posted a guide on how to unlock your Thunderbird based Athlon/Duron CPU.

The Pencil Trick is used to unlock the CPU's multiplier; I guess you could say this effectively 'unlocks the full potential of the CPU'. Say if you take a 1GHz (100x10) CPU and raise the FSB to 133 and the default multiplier is 10, that's 133 multiplied by 10. That gives you the new CPU speed of 1.33GHz. If you tried this style of overclock with your Athlon or Duron, it would be likely to run unstable and crash. By implementing the pencil trick you can change the multiplier from 10 to higher settings, such as 11.5, allowing you to take the FSB to a stable and standard level such as the default 100MHz and still have a nice overclock.

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