Requires you to unlock the Ghost stance, which is unlocked during act 2 throught the quest 'Ghost of Yarikawa'.
fully upgraded, it need 7 kills without being hit to activate it. This includes stealth kills also if you want to be sneaky about it.
Once you get seven kills, the meter on the bottom right of the screen glows red. Hit
together to start it and this mode will let you kill 3 enemies within the timeframe it is active for (sometimes if you're lucky, you can hit a fourth).
Really helpful during Mongol Territory or big set pieces.