Almost the same thing as the Dirty Sanchez, The Dirty Snachez is when you are having anal sex and pull out and put your penis back in her vagina. The thrusting creates residual poo to build up around her vagina.
Under anal intercourse, put your finger up her anus, then draw your finger over her upper lip, giving her a brown stripe resembling a mexican revolutionist mustache. Thus "Dirty Sanches"
the act of putting ones fist up a girls a hole and then taking it out and smearing the poo stained fist where her brazilian should be and then make a couple of eyes out off the poo.