Since it's permanent and is focused on your 'set' it has to involve your equipment, especially considering the whole 'understanding of thine arms ' stuff.
So yeah, best guess in my mind is something about your active equipment (armor, weapons, mount etc) is getting a slight boost (+armor, speed, maybe less requirements to wear or less upkeep?) is being permanently added to stuff. Like someone said, maybe an alternate to heirlooming.
The big question is a) is it being shown on equipment or is it hidden? b) what triggers it exactly?
It would be cool if it lowers requirements to wear items.. the set you normally wear then 'becomes' your usual set even if you retire and are low level normally.
DAMN JOO MYSTERIES!