It's called "Punish", and it makes you pimp-slap your current companion because... Well, because Sith don't need a reason to be abusive.
I made it a habit of using it on Khem Val every time he Disapproved of me making a decision just because it wasn't murderous-crazy. His sobbing is music to my ears.
I must make a Pureblood character one day. I can see myself using it a lot on Vette. Perhaps I could reach 50 and make another agent, I'd REALLY like to use it on Kaliyo.