Real Danger
I have heard of at least one case of a freediver suffering severe injury due to embolism on the ascent, but it was years ago and I can't remember much about it. Spearfisherman I think.
On my first two days of freediving, my chest was very inflexible, and even without packing, on every ascent air would explode out of my mouth in the last 2 metres. It was pretty scary.
These days, if I pack to the max, I always need to exhale a bit in the last 1-2m. The danger is far greater on shorter dives. During long dives (or long statics on the bottom of the pool), the lung volume shrinks due to CO2 dissolving in the blood/plasma/tissues. If I pack to the max, go down to 2m, and then come up without exhaling, I would almost certainly suffer injury. The main reason is that I pack 3 litres over my max inhale volume, and to get those 3 litres requires an extremely strange body position (back extremely arched, shoulders way up, head cocked way back, belly out). If I am not in that exact position, my lungs/rib-cage cannot handle the volume. Once I get down to 2m, I don't need to assume that position anymore. Then, if I (immediately) ascend in a 'standard' relaxed posture, that posture is incompatible with the volume of air in my lungs. What would probably happen is the air would come bursting out of my mouth (if I didn't exhale on purpose), but if I slammed my throat shut, you would probably hear a loud 'boom' (like a sonic boom) as my chest detonated from the pressure. There would be pieces of me everywhere!
Just remember that the greatest danger you can ever put yourself in is to pack to the max, and go down to the bottom of the pool, and then 'abort' the static and re-ascend without exhaling.
Eric Fattah
BC, Canada