How did Asajj Ventress hack Savage Opress?

If you look at this video, Asajj Ventress hacked Savage Opress merely by touching his head. This is more impressive than Jedi Mind Tricks.

But, how did she do it? Is this some Sith power as she was trained in Sith arts by Dooku himself? This is surprising because I thought Sith didn’t have powers to control others. If yes, what is its name? And, why didn’t she use this power more often?