I am not an authority on packing, but I have had a few blackouts after a lot of packs, especially earlier in the day, and if I haven't got my reflex kicked in. I just learned how this year from an earlier post by Eric Fattah. Laminar will be able to elaborate more, but it has somethimg to do with the extra presuure on the heart and carotid arteries I believe. If your CO2 level is especially low, ie after a long breathe-up, and you pack a lot of times, then you are more subsceptible to dizziness and blackout. If you start to hear ringing in your ears, tingles, warmth, and perceptual narrowing, then you are getting a warning that blackout may be imminent. At least that's what I experience. I ride it closely, but am learning where to back off.
It's not hyperventilating, and I don't think it's dangerous in itself; there is no evidence that lung expansion injuries can result, as far as I've read.
I would reccomend that you learn how to do it, and slowly keep building up to a substantial amount. If you want to dive extremely deep, then you will have to, just to able to equalize at depth. Some static competitors use packing as well; the time it takes to pack the extra air is outweighed by the gain in breath-hold, but you have to be stretched out enough to be able to do it reasonably comfortably.
Hopefully Eric F., Laminar, or Peter Sheard will reply with a better explanation of the process.
Cheers,
Erik Y.
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