zooted is right. But I have to add that not all motherboards unlock hidden cores, some of them have the function with the bright highlight in the package (mostly AMD 710SB), others still have the function in the BIOS Setup but don't announce it... and others simply don't have it. I personaly matched an Asrock N68-GS3 UCC with a Sempron 140 and actually could unlock the second core converting it in an Athlon II X2 235e. Still low end but awesome for the budget.
Later I matched an ECS 880G-M7 with an Athlon II X3 with no luck. The 4th core was unlocked and the Bios recognized it as an Athlon II X4, but always crashed loading windows, so I asume that 4th core was hidden because it was actually defective.
So its a risk, you cant count on it, but considering that you have a probability to get more than what you pay, worth it.