Maybe it's a common sense or old as my granny but I hadn't found any similar topics, if they exist pls delete this one.
So main idea of this exploit is to switch specialization from muti to any other before fight starts, so what you need is muti spec with vendetta extra time glyph and any other in which you will fight with boss.
What you must do is:
1) spec in mutilation before the fight starts
2) put on boss vendetta (fight won't start)
3) change spec for ex. in combat
4) begin the fight
5) you will have vendetta debuff on boss for 34 sec+ from muti spec (+20% damage) and be in combat spec (ex.)
still works - Video: http://www.youtube.com/watch?v=ETeeMXVFBwc