Manufacturers must enable a feature called Secure Boot in their products’ UEFI firmware in order to be officially labelled Windows 8 compatible. This mechanism will only start operating systems that have been signed with a digital key recognised by the motherboard’s firmware.
Modifying the computer’s start up process, such as installing a completely new operating system or updating the existing kernel core, will invalidate this signature and cause the firmware to reject the software until it is signed again by a trusted secret key.