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The unlocking process works with some 'Pro' SKU graphics cards utilizing either AMD's Fiji, Hawaii or Tonga GPUs, such as the Radeon R9 Fury, R9 390/290, and R9 380/285. In these cards, AMD used a cut-down version of a GPU with some compute units (CUs) disabled either through firmware modifications or permanent laser-cutting.
A tool from Overclock.net member tx12 called CUINFO allows you to determine exactly which CUs have been disabled in your GPU. Using this information, you can flash a new BIOS to your graphics card that attempts to fully or partially unlock disabled CUs. If, for example, you could unlock all of the disabled CUs in an R9 Fury, you would essentially end up with an R9 Fury X.
Naturally there is limited success with unlocking CUs, as some have been disabled because they were damaged during the manufacturing process, maximizing total yield. Unfortunately there is no way to tell if a CU has been disabled for this reason, or simply whether a CU has been disabled to create a lower-end SKU.
Re-enabling CUs does lead to increased performance in cards where this is possible, although the unlocking process almost certainly voids all sorts of warranties. While it can be an interesting experiment for an enthusiast wanting to get the most out of their GPU, unlocking CUs definitely isn't recommended for everyday users.