One of my instructors gave me a tip to improve air consumption: square breathing.
I didn’t like it.
I felt like I was following a sine wave, slowly ascending for the first 8 seconds then slowly descending for the next 8 seconds. It didn’t feel like neutral buoyancy; it felt like out-of-control buoyancy.
Also, I know what you’re thinking: NEVER hold your breath underwater!
However, based on my training, I made some logical assumptions. First, I never did a full inhale; I left space. Second, my depth only varied a meter or two during an entire cycle; although the volume of air in my lungs was changing, it was not changing much at 20-40 meters. And, third, I only tried a few cycles at a time, mostly because I didn’t like the loss of control from the buoyancy changes.
Maybe I need to practice it some more. I don’t know. But, I think I would prefer to find other ways to reduce my air consumption than to spend so much time and energy controlling my breathing.