Thief Booster?

I got the theif booster finally in my last war.  It immediately takes away 3 energy from the person who you put the boost on.  I think it gave me the 3 energy but not sure, I did not know how to use it, etc so I was just experimenting with it (I won it from VU level 2).  I had full energy when I did this so I don't know if I wasted it or if it gave it to me, etc.  My boost was a level 1 and it said 15 minutes.  Not sure what any of that meant.  I can't find anything about details on what it does, etc so this is just from my experience of using it.