When you work out you're burning fat which in return releases thc molecules. Which is good but you don't want to do that when you have a drug test coming up. Most of the thc molecules are released through your intestines(simplified explanation),So if you don't keep the old poop shoot moving the thc is reabsorbed again and relesed through your urine. So the idiea is to keep the old hershey highway flowing hence the high fiber diet.