Sentence:  As the rectum fills with the products of digestion, it signals, through nerves running into the sacral region of the spinal cord, that defecation may be necessary. The internal and external anal sphincters then begin a culturally mediated <WSD>pas de deux</WSD>, the former pressing for release and the latter restricting discharge until the opportune moment. 
Meaning of the word marked with <WSD> tags:  A complicated relationship or process involving two entities, usually persons.