Both were nice partner. They helped each other for sexual pleasure. When the sex level of a girl is very high, they want to ride on the boy and penetrate each other. So, she was doing slowly and when she felt very high, at that time, she made a sound " o yeah " . It's called high level moment and her dopamine level was very high at that time. She helped her partner. Otherwise He couldn't perform as some time. It needs to help for his partner