When you eat nothing but power bars and seltzer water for a week, causing your shit to be thick and bubbly, hence the "pop."
After such a shit is obtained, the shitter takes a partial poo, keeping some around the hole (similar to a juicy rusty trombone) and a second person eats it with a spoon. This is usually done before noon in honor of Rice Krispies (TM)
Kate: "I love eating Rice Krispies (TM) for breakfast! They go 'crap snackle pop!'"
Peter: "Kate, you fucked that one up. But now that you mention it...would you eat the shit crust from my asshole?"
Kate: "Wow, that tastes so much better than cereal! MMMMM!"