According to Jam!, Rollins told Sun Media that a studio engineer let him in on Britney's dirty little secret. "They have the black chick come in and sing, and Britney sings over it, and they mix them together," Rollins said.
"(Britney) gets her phrasing basically from this older R&B woman. I found that out talking to an engineer. Britney apparently isn't actually the worst singer, she just has no feel. So they bring in this older black woman who sings the song, then Britney sings to it, and they kind of make a mix of the two voices, and that's what you hear on the records."
The quotes in this article came from a - Jam! report