Reasons for choice of acupressure point GV 26 (just below the nose and above the upper lip).
1) This zone can be seen to be drawn tight when a person has to think hard or has to make some physical effort. Pressing on the point would be a more natural approach.than one recommended point on the chest such as lung 7, which did not work for me years ago in my asthma time. GV 26 has a very strong effect for me.
2) Failure to get GV 26 to work might be the difference between an asthmatic and a non-asthmatic.
3) Generally it seems that mouth breathing with slackened lips greatly decreases lung function.
4) The strong effect of pressure on GV 26 with an accompanying effort to inhale must come as a surprise to many and the fact that many folks with acupressure in mind would first try specific lung points on meridians might explain the comparative lack of success with asthma and acupressure. Richard Friedel

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.

For a detailed list of references see the bibliography of 138 titles (1974-1998) which documents the effects and clinical efficacy of stimulation of acupoint Renzhong (Jenchung, GV26) in humans and animals on http://www.med-vetacupuncture.org/english/articles/gv26.htm
This acupoint concerned here as an asthma remedy

As regards the popularity of the technique with the acupoint GV26 (between the nose and upper lip), see WorldWideScience.Org. GV26 has 469 hits.
As regards the saftey of the method, see http://www.sciencedirect.com/science/article/pii/S2211766014000231 That is fair enough.
The point is firmly and attentively massaged and the chest is unlocked by reflex.