The sphincter muscle is like a drawstring, which is why you want to make sure it is relaxed and go slowly. If you tear it, it makes it difficult to hold...you know. If you are not careful and do research, you can accidently scratch or tear someone inside which can cause infection, among other things, even with your fingernails. With patience and knowledge, it's low risk. However, if something hurts, then stop.
And no, I have never been fisted in any manner.....laugh. Just a medical background.