Apparently, by just replacing imgfs.bin instead of trying to build a ROM from the ground up, chefs can bypass the many Genuine Advantage protection mechanisms Microsoft has incorporated into the platform.
However, before you go hooking your Focus or Quantum up to the PC, keep in mind that these images probably won't even work on non-HTC devices, as the drivers seem specific to the Taiwanese OEM's hardware. Furthermore, even those HTC WP7 owners probably don't want to attempt this, as you won't be gaining any additional features or functionality out of such a hybrid ROM; basically, this is just a proof-of-concept that should eventually lead to custom ROMs once chefs have a better understanding of, and more control over, the packages included in the various ROM subsections.
Source: PocketNow