The V64 unit contains a CD-ROM drive which sits underneath the Nintendo 64 and plugs into the expansion slot on the underside of the Nintendo 64. The expansion slot is essentially a mirror image of the cartridge slot on the top of the unit, with the same electrical connections; thus, the Nintendo 64 reads data from the Doctor V64 in the same manner as it would from a cartridge plugged into the normal slot.
The Doctor V64 could be used to read the data from a game cartridge and transfer the data to a PC via the parallel port. This allowed developers and homebrew programmers to upload their game images to the Doctor V64 without having to create a CD backup each time. It also allowed users to upload game images taken from the Internet.