I'm the one who wrote the deep diving stories in which I say that I did pack-stretching and packed 43 times before descending. Pack stretching and packing are very similar. They are a technique in which you can force more air into your lungs, even after you inhale to your maximum. For example, my lung capacity without packing is 7.8L, but with packing it is 10.2L. With a wetsuit on, without packing, it is 7.0L, with a wetsuit on, with packing, it is 10.2L. So I get 3.2L extra air from packing. Almost all competitive freedivers know how to pack. However, be careful, because in the beginning your lung tissue isn't very stretchy and you can damage your lungs if you pack too much. Once you have inhaled to your max, suck air into your mouth as if through a straw. Now close your mouth, and use your cheeks or tongue to push that air into your lungs (not your stomach--don't swallow the air). That is one pack. Pack again and again until your lungs feel full and stretched, but stop there.