Defaecation is a complex process that involves interaction of neural processes & reflexes on one hand with colorectal contractility and biomechanics of straining on the other hand,

The process is simplified in the following Flash animation. Basically the brain centres for defaecation respond to the sensation of rectal filling & stimulates the colon & rectum into mass colonic movement, normally once per day. These mass contractions propel faeces along the colon & rectum, occasionally helped by some straining. Straining under physiological conditions result in transmission of forces to upper rectum, aiding defaecation

Straining becomes more important [but chronic long term straining can result in problems of internal & external prolapse] if faeces is hard, if mass movements are ineffective or if rectal & sigmoid configuration interferes with easy propulsion of faeces. Patients with ODS eventually resort to too much straining, which may become counterproductive if it increases rectal lateral bending, internal rectal prolapse or if it is transmitted to lower rectum