This is a hack to prepare your unit to install a map/firmware update. Confirmed to allow firmware updates for all Z110, Z120, Z130, Z140, X920, X930, X940 - and probably their corresponding rest-of-the-world units. I can't say whether it will work with the very old units, and it won't work with the model numbers ending in 50 or greater, or Nex units.
This should work with the following F series models:
F10BT, F20BT, F30BT, F40BT, F840BT, F8430BT, F920BT, F930BT, F940BT, F9210BT, F9220BT, F9310BT
Abstract:
This was an early method used to hack map update releases.