.....The danger is that if you breath from the bag and then rise to the surface with out exhaling, your lungs will burst.
........You can feel the pressure building in your lungs as you rise and you automatically exhale.
By the time you actually feel the pressure in your lungs, you've long passed the critical point.
Gas(air) expands as pressure decreases when you ascend, but is not generally a risk of bursting your entire lung like a balloon. It's the little sacs at the end of the system, where the blood-air exchange takes place, the alveoli, that are the concern. These can rupture with virtually no warning feeling whatsoever and introduce infinitesimal unsaturated air into the blood stream, which will not pass through the small capillaries and, therefore, block blood flow. If this happens in your finger, you probably lose feeling in it. If it happens in heart tissue or the brain, you may die. You can't choose where the air embolism occurs.
Air will not likely expand sufficiently from 2 ft below the surface to cause this, but theoretically could. It is a real risk from as shallow as 6 or 8 ft, without proactive exhalation. I didn't want to leave anyone the impression that they would be okay by automatically feeling the need to exhale. In fact, its a natural reaction when you are scared to hold your breath. It has to be trained out of you.
........ former deep wreck rescue dive instructor.......