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Because constant finning will use more air than will a simple change of breath. Look at it this way, your leg muscles use a lot of O2 and if you push them all the time you're going to use more air to support the movement.
When you get to dive with a great diver like I've been privileged to, you'll notice two things. First they don't fiddle with their inflator(s) and second they seem to do things without apparent effort. That lack of effort comes from good buoyancy control which your lungs help control. Keep in mind this is for short excursions, not a 10 minute trip down 50 feet.
The other key IMO to good air consumption is to stay horizontal, by using your lungs to help buoyancy and not finning you maintain that horizontal orientation. If you're finning down you head is low causing more drag, if you're finning up the same thing occurs. The faster the motion the worse the drag, and the worse the drag the more energy you put into maintaining it and to output that energy level you need to burn O2 and that means you're going to breathe more air. It's a cycle.
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Matthew P. Cummings
Moberly MO
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