Backstab II  

Line:Critical Strike
Level:10
Realm:All
Prerequisite:Behind Target
Attack:High Bonus
Defense:Medium Penalty
Fatigue:High
Damage:High
Effect:Stunned
Effect Information:
Stuns the target for a brief period of time. Resist type: N/A Duration: 3 seconds.

Also, this style requires you to be stealthed in order for it to work.