She feels like she should...know exactly what that is. But her brain is a little frazzled right now, cloudy. And it's obviously something sexual, which (not that Rachel Berry would ever admit not knowing much about anything) she doesn't really know much about. Discussing sex was just something she'd never done. But she doesn't ask Noah to clarify what exactly "sixty-nine" is, for fear of him laughing at her or something equally atrocious. So she just bites her lip and nods.

"No, never." She stills, afraid she's doing it wrong since it's that obvious. She's about to unwrap her fingers, Did I hurt you? just breaking off of her lips when he wraps his fingers around hers and doesn't let her pull away. And if he's willing to teach her, then she's always willing to learn. Rachel Berry strives to be the best in all of her endeavors, of course, whether it be dancing or singing or having sex and what better teacher could she have than William McKinley's resident expert?