[jome]

Personally I think training effect vise the most beneficial is not hyperventilating for several reasons. The biggest being that you can reach much lower O2 saturations without it before passing out (bohr effect etc) and it would make sense that it also thus rewards the most training response and adaptation. It's also mentally more demanding, thus effectively teaching that aspect of apnea as well.

Dry training is great for experimenting with the amount of hyperventilating that offers the most benefit. If you don't know you "optimum breathup", I would keep a diary of attempts and try different kind of ventilation strategies in between absolutely no breathup and mild hyperventilation (personally I regard 10x5:0:10 maximum, after that I start to get clear symptoms of ventilating too much. Optimum for me seem around 5x5:0:10). It's relatively safe to make such experiments, but I would still encourage using common sense and not attempting huge leaps or extreme hyperventilation. And it never hurts to at least have someone aware that you're doing breath holds...