What matters is inspiring with a fair amount of effort. On switching between an effortless inhale and a vigorous one a distinct difference is sensed on applying pressure to the acupressure pont between the nose and the upper lip. There is a smart natural reflex, like blinking when dazzled, which guards our eyesight.. In practice one would breathe with a stiffened or tensed upper lip.
Breathing through the mouth with slackened lips does infact greatly impair lung funtion, so it seems that Chinese medicine provides a better understanding of asthma than western mainstream medicine, which does not cure.