It is designed to increase the width of your airway. However, IIRC performance is limited by the amount of O2 delivered to the muscles rather than increasing the volume of air to the lungs. I can’t remember the article but, it basically stated we can suck in more O2 than we can use already. The real magic is more RBC’s…
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These types of breathing things should be looked at with as much skepticism as you can muster.
Others have already said it: lung capacity isn’t the limitation… delivery from the lungs to the muscles is.
One thing some breathing restriction devices CAN help train is your breathing muscles which can fatigue over a long time like any other muscle… but on the list of things to worry about training and buying something to specifically train for XX minutes per day, it’s somewhere right around training big toe strength to gain .1% on your upstroke IMHO.