The obvious muscles would be the adductors: semimembranosis and semitendinosis. But the melon would block those.

She's got to be using the internal hip rotators. And the only way that works is with the heels locked together.

I'd be impressed if she could do it without locking her heels together. As it is, I'm not.

Oh, and Box, if this is a he, that's impressive. You don't want to match male and female hip adduction strength. We don't do well...