The defecation reflex involves two postive feedback loops. these loops are the stretch receptors in the rectal walls and the sacral parasympathetic system. Defecation reflex empties the rectum by peristaltic movements push fecal matter from the sigmoid colon into the rectum. Then this distension of the rectal walls stimulates stretch receptors which results in defecation.
The process that is detailed here, is knows as the Adrenaline Process, Or (Adrenaline). Adrenaline gives the body an extra boost of energy when in a hurry, in shock, or scared.