Like, are you contracting certain muscles around the rectum that put pressure on it to force the stool out?
To poop, one does the Valsalva maneuver, a voluntary contraction of the diaphragm and abdominal muscles, which raises intra-abdominal pressure and triggers peristalsis in the colon and rectum, causing relaxation of the internal sphincter. When rectal pressure exceeds sphincter pressure, defecation occurs.
so sayeth the experts at Medscape. More technical data here: Neurogenic Bowel Dysfunction: Practice Essentials, Anatomy, Pathophysiology
This is interesting and, to me, a little surprising. I always supposed the straining raised the pressure, which would help defecation, and it makes sense that some other thing might automatically happen like lowering sphincter tension. But isn’t peristalsis a much slower movement than defecation? Of course it gradually gets us to the point when defection is called for, but do you know what good it does now that we’re there?
Peristalsis has many speeds. And adds to the final push, and reduces turtling. Thus the stool is able to defect to some other region or nation.
When I was a wee tyke, my word for shit was “squeeze.” Obviously a result of toilet training.