When a man has anal sex with himself he forms a Candado Chino, in spanish a chinese lock.
Logisticaly it is not easy to accomplish, common belief is that the flaccid penis is tucked between the legs and fed into the anus then stimulated into an erection.
- Mate that dude did the Candado Chino.
- What? You mean he fucked his own arse? Gnarly...