Isaac made a happy, choked sound, as Derek's thumb slipped over the flat of his tongue. He didn't sit back and let in happen, but instead he pushed forward, trying to force the digit as far back in his throat as possible. He would have been happy to gag and choke, if that was what his Master wanted.
As his mind focused on Derek thumb, Isaac's hands seemed to move on their own. He gripped the shaft of his cock tightly, working the foreskin up over the head, and back again. Without the focus to slow him down, the strokes were swift and hard. It wasn't purposefully so, but it was just how Isaac needed it.