Asthma causes both inflammation and constriction of the airways. Asthmatics air-trap because it's easier to actively entrain air into the lung than it is to passively exhaust air out. Watch an asthmatic breathe and you'll see active, forced expiration. After awhile that trapped air has lost all of its oxygen and is now largely carbon dioxide.., and it's displacing fresh inhaled air. The PEEP of +9 would be to splint open the smaller airways so that the trapped gas can get out.
The other night I had a pt who was thought to be an asthmatic (a diagnosis I doubt) who had worsening respiratory distress. In an effort to avoid intubation the doc decided we'd try CPAP of +10. But the kid was so anxious and irritated by the mask and pressure that it didn't have a chance of helping. What did help was Ativan! 12 hours later after no albuterol, cap gas was perfect, RR was in the 20s, sats in the high 90's (still on significant O2, but there was some pneumonia on CXR) and work of breathing virtually nil... hence my doubting the diagnosis of asthma. I apologize for hijacking your thread.