I'm not quite sure, sometimes simple things go over your head, but I am almost certain that the background sample vocal for this is "My Neck, My Back."
The girl says "Do it, do it, do it, do it, do it, do it now," and "My back, my back, my back."
Haha, check out the sample I think it's from then check out the song, and you form your own opinion. I could be just crazy, but when I hear vocals like that, I assume their sampled, and I automatically match them with the first thought in my head, which was "My Neck, My back."
But whatever, don't let that stop you from enjoying the song! Peep it. Peace, wolves.
(Assumed sample, below. WARNING: VERY EXPLICIT LYRICS!!)