Yes, you heard me correctly. Fear will bring about the desired potty use. It's simple really. You just use your child's natural fear of heights to work against them. Something like this:
You tell your precious little darling that since they have not been using the potty, you're now out of diapers. Then take your non-potty using child with you to the store to shop. Once there, tell them that since they're the ones using the diapers that they'll have to be the one to get the diapers. From the top shelf. Which they can't get to unless they are hoisted up there and left for a bit. Just long enough for the panic to set in. "Oooo! It sure is high up! I hope I can get you back down again!" Yes, the fear of diaper shopping will get your potty-avoiding tot sitting on the throne in no time.
"But Wendy," one might ask, "what if my child isn't afraid of heights?" Being the fount of wisdom and all things parenting, I have an answer for that, too. Two words for you: Krazy Glue. You're welcome.