Is there a video tutorial for this Vocal Effect

Hey there
Im looking for tutorial of this effect, please listen;
https://www.youtube.com/watch?v=LqDsLaWTen0&feature=player_detailpage#t=7
Theres a sound coming behind of Britney's vocals, like a ghost, or a monster.. Call it what you want! LOL.
Can you teach me how i can make this one for my vox?

If it's what I'm hearing it's a synth behind the vocal. You could likely get the same result by using a detuned saw-wave low passed with a slow portamento sliding down a octave.