If you hold your breath and move upward, even a few feet, the expanding air can overinflate your lungs and cause a serious injury called pulmonary barotrauma. That is why instructors simplify the message to “never hold your breath.” It is a shortcut meant to protect new divers who might not yet understand how pressure changes work. I would rather take the extra ten minutes to explain the truth than give you a slogan that you will later find out is wrong. You deserve to understand the reason behind the rule.