Is this just for your own edification or are you intending to build something to interface with it? Search the board for "HD20". You'll find a long thread that contains a bunch of links describing how the Apple floppy drive system works. The most unique aspect of Apple machines is how drive head stepping is performed. (Links in said thread describe it. Note that Apple used both more-primitive and more advanced systems than "normal" Shugart floppy drives on the Apple ][ and the Macintosh respectively.)