When a person really has to shit and he builds up pressure inside, then just when he feels like he is about to explode... he lets out a pressure relieving fart, which gives him about an extra 30 minutes until the pressure is built up again.
Justin: "I HAVE TO SHIT SO BAD MAN!!"
Riva: "cant you hold it?"
Justin: "NO IM ABOUT TO EXPLODE!!!"
Riva: "okay lets go to the bathroom then"
Justin: *extensive fart*
Justin: "Nevermind - I just reached a checkpoint."