Complex sentence: Dumbledore advises Hermione and Harry to use Hermione's time-turner, a device she has been using to double-up on classes, which permits them to go back in time and save Buckbeak, who carries Black away to safety.
Split into simpler sentences: Dumbledore advises Hermione and Harry to use Hermione's time-turner, a device she has been using to double-up on classes. This permits them to go back in time and save Buckbeak, who carries Black away to safety.