For the treatment of asthma sympathomimetic drugs can be used to
emulate the effect of the sympathetic nervous system.  When stimulated
it opens up the lung airways, as can be seen from a medical textbook.

As clearly proved by the success of Oriental Martial Arts breathing on
the tanden (a point an inch or two below the navel) can have a strong
vitalizing effect. This is said to be due to stimulation of the
sympathetic nervous system.

The fact that descriptions of such breathing practices use unfamiliar
terminology (chi, prana or ki as meaning oxygen and life force) should
not deter people, because it is also a question of tuning the nervous
system, not just gas exchange.

One elementary method of generating the breath of fire as a path to
tanden breathing is bhastrika, breathing vigorously and quickly
through the nose with due emphasis on diaphragm action for about 30
secs.  Although it may well lead on to slow diaphragmatic breathing it
might well be unsuitable for the average asthmatic without preparatory
training because of the hyperventilation phase caused.  The method is
unsuitable for those suffering from high blood pressure. Bhastrika is
recommended for asthma by the yoga folks. Of course treatment and
instruction should only be sought from a responsible expert familiar
with the brittle nature of some asthma.

My personal experience of breathing with a concentration of "energy"
on the tanden is that it causes a natural, pleasant sensation with a
power over airway constriction and over vitality. Respectfully,
Richard Friedel