How does Nesta show the priestesses a demonstration of what training with Cassian will be like?

Nesta is practicing her training moves as she shelves books in the library. Clotho asks Cassian to tell her to stop. When Cassian finds Nesta, she is practicing. He corrects her form. Then she has her do a few moves. When they finish, they notice that a group of priestesses are watching. Nesta will no longer need to practice in the library, she has shown the priestesses what she is learning and how it will be like to work with Cassian.