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I'm starting to see that if you're not being instructed in a certain amount of pressurizing, then you're *probably* not getting the real goods
Your observation is correct. Therein lies the mystery. How to you exhale, regulate, and "relax" into it. Likewise how do you regulate on the inhale, creating the vacuum, and relax at the same time.
Vacuum & Compression: this is what a carborator does and what a turbo charger does in a race car.
Paradox: how to long breath, and forget yourself, at the same time. Any point of tension (to regulate the breath) draws the mind to itself in meditation, so regulation has to originate from an organ associated with the breath, less it be extra baggage on your "trip" that you drop along the way.
Spectrum
ref: Condensation Breathing