Whether it is constipation or diarrhea, irregularities in bowel movements on some level all relate to the balance of fluid in the intestines. Fiber doesn't just add bulk to feces, it holds onto water. If you go all Linus Pauling and take way too much vitamin C, you'll get diarrhea - the prodigous amount of dry water-loving solute you are ingesting will draw water into the intestines.
Many laxatives, such as sodium picosulfate, work the same way. Water follows ions, and this doubly-charged molecule draws water into the intestines, with the result of inducing a bowel movement.
If you are unsure of where your feces line up, the helpful Bristol Stool Chart may prove instructive.
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[Correction] - I need to correct this post. Sodium picosulfate is not an osmotic laxative, it is a stimulant laxative. it is usually only taken in the amount of 10mg or 20mg, hence would have a miniscule osmotic load. It stimulates peristalsis/contractions in the colon by causing neurotransmitter release from the gut enteric nervous system and hence stimulates GI tract motility. What acts as the osmotic laxative is the ~30g of magnesium citrate that is taken with the sodium picosulphate. That would draw water into the GI tract and cause it to stay there. A better example of an osmotic laxative would be PEG 3350 (although MW can vary). It is non-absorbably osmolyte that remains in the GI tract causing water to be retained. The ascorbic acid example that you use is also very good. Vitamin C is only partially absorbed in the GI tract, so a large dose ~20g, only 2-4 g would be absorbed and the rest would remain in the lumen of the colon acting as an osmotic laxative causing diarrhea.
Sincerely,
Prof. Diarrhea (GI epithelial physiologist)