It puts pressure on the spring that holds the valve closed until you inhale. The reg should be set u7p so that with the knob screwed out as far as you can screw it, it will breath well and not leak. If you screw it in, it will make it more difficult to breath. If the reg leaks a bit wityh the knob all the way out, you can screw it in slightly to stop the leak. Such a leak may occur as the seat wears or as diving conditions change. Current, surge, or even very cold water could cause a slight leak that you can manage with the knob. It is also useful if you use that reg as a backup and you want to make it less likely to leak when it is not in your mouth.
