Ones that don't arch back and part knees? Bro,  be a man and push the small of her back down while rotating her hips with your other hand.  ANY person with half a brain will understand what you're trying to get across.  And for God's sake,  don't ask her or assume she'll part her knees,  fucking tell her to or use your own legs to push them out of the way.  Does this guy not sex?