It is a matter of training. Asanas do their part by helping to ease the congestion in the lungs. This promotes relaxation of smooth muscle and better oxygenation. The change, however, does not happen overnight, as asthma is a chronic disorder and irritable responses take time to stabilise. The earlier the asthmatic begins yoga the better the response.
Constant practice is required to change the sensitivity of the body.
The practice of asanas during an attack is different from when the patient is free of symptoms. All asanas are useful. If the patient is stable, standing poses can be practised. Back bends are very helpful to open the frontal lungs and relax the bronchial muscle. Props may be needed.