There are also girls that double slap. They are actually hitting the back of their hip, too. If they don't stop doing it, I don't work with them very long. I don't want people to think that I teach that. I tell them, to lead with the glove and not bring it down. If they are slapping as their hands go back, I just tell them to miss their hip.
My own DD started pitching like that and the coaches and I just decided that we wouldn't let her pitch, if she was going to insist on bruising herself and ruining her wrist. She stopped on her own.
Cat touches her hip, but the girls that I see (and I had one pitch for me in college) are making a loud popping sound that results in a glove sized contusion. Good luck.