The wetback shit is the act of taking a shit and sneezing. The sneeze propels your shit out of your the anus at a higher than average rate. As the shit licks the surface of the water, it causes a large ammount of water to discharge from inside the bowl into your anus thus resulting in a wetback shit.
Man 1: Last night i was looking to take a peaceful shit. I brought my magazine in with me as usual. Suddenly I had an urge to sneeze. I had been prairie dogging it for a good four hours previous to this. As i sneezed, the shit propelled out of my anus and i received The Wetback Shit.