i recently noticed that many amd motherboards come with an "unlock hidden cpu core" option
then figured out some processors really come with hidden cores
now why in the world would they make a core. and then hide it..? i have absolutely no clue
Because they only make quad core CPUs, its much easier to mass produce a single CPU design than to have the fab making 8 or 10 different ones. None of the chips come out defect free, those that have a defect in one core become an X3, this defect might be a critical flaw which keeps the core from operating properly, or it might just not be able to maintain the speed at the same voltage so they disable it so it doesnt cause the user any issues, this way even though its slightly broken instead of throwing it out they still make money off of it.
During the early days of manufacture in a new quad core design, example the Phenom II quad core, the yield for the chips with 4 perfectly good ( for the specs ) is low. So rather than throw these chips away, they sell these as X2 such as the Phenom II 550 and the X3 such as the Phenom II 720 to get back some of the expense that came in the manufacture of these chips.
Now during these times, when these X2's and X3's are sold, they really have defective hidden cores. But they are cheap and comes out better than existing dual cores so people buy them.
However, as these X2's and X3's become popular, the demand for these are no longer met by the Phenom II chips that comes out with defective cores (specially since the manufacturing process improve and less and less chips comes out with truly defective cores ), therefore, to meet the demand, AMD intentionally lock or hide good cores so that it can be sold as X2 or X3.
As people discovers that they can be unlocked with the ACC function ( now this feature is expressly labeled as "unlock cores" function ) in some motherboard, the demand for them become even more greater... thus, the cycle begins so the chance of unlocking good cores out of the X2 and X3 are higher, though still not perfect.