"It's called penetration denial," Shay says.
"With penetration denial, I would bring him right to the edge of an orgasm while his penis was inside me, in my vagina. It was hugely frustrating for him, since it felt so good, but he would have to hold back his orgasm unless I gave him permission to cum."
At this Crystal seems to have a huge orgasm, although you wonder why. In your memory Crystal rarely seemed to have orgasms when you made love to her.