He had to be careful, if he moved too suddenly she would cum. So he carefully touched her - but didn't move - just to make sure she wouldn't finish instantly.
She rocked her hips, biting her lip hard as near maddened laughter tore from her, her juices heavily coating his fingers as she climaxed, nearly on command.
He didn't know why that made him as hard as it did, but he wasn't about to deny it. He slowly stopped and pulled his hand away, giving her time to gather herself.