Diarrhea is an acute response to some problem in the gut, and in fact it is your body attempting to cleanse itself of some irritant or toxin. That most certainly is not a myth. If you just have a couple of loose movements over a day or two you're probably experiencing a run of the mill response to something you ate. Stopping diarrhea via some intervention in that case can actually prolong the problems you're experiencing by ensuring that the trigger remains in your body for longer.
If it becomes chronic or is very severe, then you have a more serious problem and need to seek actual medical attention.
I just fast and try to get enough water. If it's a really bad bout that lasts over 12 hours I might drink some broth and/or coconut water just to hack my hydration levels. If I know what caused it (drunken dietary indescretions, food additives, cheats) than I might try to accellerate its passing with some probios and kimchee (for the fiber and enzymes).
More than 24 hours? I'm going to the doctor. Found out the hard way that trying to man up through three days of the trotskis (the ceviche wasn't the best choice... ) is probably evidence of a parasite.