It was in the book, as I recall, but it didn't really make sense; Eliezer would never allow it.

So, it makes sense that if the thing falls, it could conceivably turn over and that activates the charm. But this seems really unsafe in general (I did a backflip while wearing a Time-Turner; oops!), and there's still the limit on how many times you can do it in a day, and you could just catch them, and so on.