I started yesterday! I’m guessing that because the first three days of ‘fasting’ requires minimal calorie intake it may not be good to burn off most or all of your intake with high intensity exercise leaving you with no fuel left in the tank of you know what I mean? Whereas from Day 4 onwards you are required to eat more protein and veg therefore giving you have more sustenance for higher intensity exercise? Hope this helps. It’s just my understanding ?