You can poop manually by pressing Use (defualt E key) after your gut growls to indicate the urge to go. If held too long, nature will decide when you go. Eating any kind of food will hasten the need to poop. Feces can be eaten to restore half a heart or can be used to create a Pile of Feces which causes nearby players damage from lack of breathable air. Also prepare Laxatives from Raw Digestive Agent to evacuate your bowels rapidly.