The NANDLoader is a small bit of code in every channel that initializes the [Broadway] and then jumps to the main function of the executable. It is an example of an apploader, used for channels.
On the Wii the NANDLoader is included in a title's main binary ( app, in dol format). It is usually the first text section and should always be written to 0x80003400 (PowerPC reset vector)[check]. The NANDLoader should initialize the PowerPC and jump to the binary's main function.
On the vWii, because the Espresso only boots Ancast Images, the NANDLoader must be signed. Nintendo solved this by adding titles 1-512 and 1-513 as NANDLoaders. When launching a title, IOS will start these NANDLoaders to Initialize the PowerPC. These NANDLoaders in turn load the title's main binary and jump straight to it's entrypoint, skipping the title's NANDLoader which is included in title's binary.
Because the Homebrew Channel only had it's own NANDLoader, and not had it's entrypoint set, the channel was initially not compatible with vWii.fail0verflow solved this by adding an entrypoint to the dol header. This resulted in the channel's NANDLoader being run on the Wii, and skipped in vWii