Drops rarely while using Kota Kon to kill Burlap Trail hozen for the daily quest [88 Daily] The Burlap Grind in Kun-Lai Summit. It will take some time (far more than the 100 kills the daily quest requires).
It begins the Brawler's Guild Challenge Card quest  Boom Boom's Fuse.
Patches and hotfixes
Patch 5.3.0 (2013-05-21): Added.