Wikipedia is a beautiful encyclopaedia that try to cover all the themes. For instance, I found this article about the Autofellatio.

We learn that :

While few human males are both sufficiently "well-endowed" and flexible enough to perform the frontbend required, increased flexibility achieved via physical training such as gymnastics, contortionism or yoga may make it possible for some

There are even people that make illustrations for the biggest encyclopaedia on the Web :