system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a regulator work? (Scuba Diving) Going down 15ft without one feels like your head is going to explode, but with one you feel fine..?
gpt
It isn't the regulator that helps. The feeling you describe of your head going to explode is the air in your sinuses and inner ear shrinking due to the increased pressure as you dive deeper and deeper. Unless you equalise that pressure your ears start to hurt as the pressure outside your eardrum is greater than the pressure inside them so it feel like they are being pressed hard and it hurts.

When you dive with a regulator you remember to equalise by holding your nose and blowing against the closed nostrils as a way to make the pressure inside and outside your head spaces the same. It is part of the training all scuba diving students have to go through. If you did the same thing on a breath hold dive you would also fix the problem.

Because a reg is providing you with a steady and comfortable stream of air to breath you are more relaxed and equalising becomes easier to the extent that some people do it without thinking. Conversely, on breath hold diving, people can be more tense and not want to waste the little air they have equalising. That would be a mistake.