Question on mastery "Battle Trance" in ferocity tree

the mastery (battle trance) states "Gain up to 3% increased damage over 3 seconds when in combat with enemy champions" however it does not say it refreshes during combat or if it has some kind of cool-down. The question is what is its exit stage like? does it keep constantly refreshing in battle then going on a time cool-down afterwards or strictly 3 seconds then goes on cool-down and for how long?

I ask because i see a lot of people pick "Double edged sword" in its placement which i would assume its active all the time but i never really see anyone pick battle trance.