They are called BIOS saviours normally. They basically plug into the BIOS socket, the BIOS chip then sits in that and you copy your original BIOS to the saviour. Saying everything goes belly up and your BIOS goes down, flip a switch on the back and hey presto the saviour is used as the BIOS, then from that you can reflash your original BIOS, filp switch again and your back on the original BIOS.
A very complicated way of explaining such a simple thing lol.
Boardy