Diaphragmatic breathing is often suggested as training to cure asthma.
Here is an attempt at a proof. A popular acupressure maneuver for breathing more deeply is to press on a spot between the nose and the upper lip (see Youtube video by Dr. M.R.Gach gv26. Just type gv26 when in Youtube)
The dependency of this maneuver is on whether you as a non-asthmatic whether you chest or belly breathe with the mouth open.
1) When shest breathing with an open mouth the positive deep breathing effect is quite distinct.
2) When diaphragmatically breathing the maneuver does not make any difference.
Therefore, based on my anatomy and sensations, effective diaphragmatic prevents asthma.