Judging from you other postings, you seem to be asking why in asthma it
is more difficult to breathe out than in.

The classic answer is that the airways collapse under the effect of
forces acting during an exhale.  To prevent collapse pursed lips
breathing is recommended, which is however said not to work this way,
by simply maintaining a back pressure (see Bartolome R. Celli vol. 103
No. 4 April 1998 Postgraduate Medicine.)

Consideration of pursed lips breathing would seem to be a practical
answer to your question. Regards,  Richard Friedel