Will senokot (senna) help hard to pass stool already formed or only stool forming thereafter? if it can do the first, please explain how. thank you.

By nerve stimulation: Senokot contains "senna" which is converted in the intestines to senna aglycones. These molecules stimulate a nerve network called Auerbach's plexus which, in turn, sets in motion a series of wave-like contractions called peristalsis. It is peristalsis that moves the stool along and results in a bowel movement, usually within 6 to 12 hours. (http://www.senokot.com/constipation/treatment/)