I recall years ago, sitting in my dive class, listening to my instructor emphasize over and over again to NEVER EVER hold your breath while Scuba diving. I remember telling my husband that this would be the one mistake that I would probably end up doing. I questioned why it is okay to hold your breath while free diving and not while Scuba diving. I heard this question asked many times by divers since then. So, I feel better knowing that I wasn't the only one wondering.
The answer is pretty straightforward. When you take a full breath and dive underwater (free dive), the air inside your lungs compresses as you go deeper. As you come up, that same air expands proportionally. By the time you reach the surface, the air in your lungs is back to the volume you started with, assuming you held your breath the entire time and didn't let any air escape. It's a simple process: When you're your free diving, air in your lungs compresses as you go deeper, and that same air expands back to it's original volume as you ascend.
While scuba diving, it's different. As you breath from the regulator, the air in your lungs is replenished with every breathing cycle (exhale/inhale). Your lungs are constantly refilled with ambient-pressure air. So when you decide to surface, or you unknowingly float up a few feet, you are starting the ascent with your lungs filled and expanded as opposed to compressed like a free diver's.
Remembering to breathe normally is easy when the regulator is in your mouth. But when it isn't, (either you have purposely removed it or it has been accidentally dislodged). Most people's first instinct is to hold their breath, just like when free diving. This is extremely dangerous when your last breath was from a scuba tank.
When ever you're underwater with the regulator out of your mouth, immediately begin to slowly exhale small bubbles and make a low purring sound like "OOH" or "AAH.". Practice this until it becomes an automatic response.
Remember the first rule of Scuba Diving:
NEVER hold your breath!