When Amelia yielded, Harley steps back and lets herself drop to the ground. Because of the adrenaline still in her system, especially after letting herself be violent for no reason, it has a little side effect on her. The Joker Tokin in her body takes control, and she laughs to herself. Hopefully Amelia doesn't think Harley is laughing at her.

It might just be slightly unnerving as the laughter continues for a few moments. Amelia is not the only one who needs a few minutes.