Seen it a few times, although not so much recently... An app would pop up a rating dialog, asking you to give it 1-5 stars. If you chose 5 stars, it would send you on to the Play Store to leave your rating - if you chose 1-4, it would instead give you an in-app feedback form. They couldn't force you to actually leave a 5 star rating, but it was pretty dodgy anyway.
