allegedly it's a low friction additive that coats the gears inside the box with molydenum compounds to improve the shift and quieten the box down.
I have read up on a few of these additives before, and found that most people don't recommend putting it in older boxes that use a lot of phosphor bronze components, as it can destroy them.
it makes a similar claim to all the other additives out there - not sure if there is any real benefit to be had, or if it's just in the mind, although the garage next door to us swear by the stuff for noisy diffs.
have a read here