However, I dont know enough about it, can someone please give me a fairly short, but clear way to describe it to listeners who know nothing about freediving on the whole.
I do not know what information exactly you are looking for, but your listeners may want to know that air contains 21% of oxygen (it is ~20% once you start inhaling it, because of the added water vapor). It means 5 times less oxygen than if you inhale pure oxygen.
So hypothetically you could suppose being able to hold your breath 5 times longer than when inhaling air. That would make ~50 min at the best ones. Of course, that's not possible, because there are several physiological factors playing against it. First O2 for the breath-hold is stored not only in lungs, but also in blood, and since blood cannot be saturated much more than it already is when breathing air, you will not gain 5 times here. But most important factor is probably so called Bohr effect that describes saturation of blood (hemoglobin) with O2, and which is greatly influenced by the content of CO2. Simplified told, the higher CO2 level, the easier blood releases O2 in cells of the destination tissue, but also the less ability it has to bind oxygen in alveoli. It means that with progressing apnea (growing CO2), it is more and more difficult to bind the oxygen that is still remaining in lungs. And since the apnea under O2 is much longer than with air, and the body produces CO2 continuously (around 200 ml/min in rest), the level grows and makes the transport of O2 more and more difficult despite having still plenty of O2 in lungs.
Prolonged hyperventilation before the O2 apnea is the way to avoid this effect. Hyperventilation lowers the CO2 level. Normally (with air) it is not desired, but with breathing O2 it is practically obligatory. So achieving long O2 breath-hold is not comparable to air breath-hold, because you fight completely different physiological factors (although they are related, of course). In plain apnea you fight consumption and hypoxia, in O2 apnea you fight especially the hypercapnia (which then may cause hypoxemia too). Also CO2 poisoning (yes, CO2 is toxic at higher concentrations) is a risk at O2 apneas.
So in fact the length of record O2 apnea is currently still very far from the hypothetical limits. Unlike at normal apnea, it is more about finding the right degree of the pre-hyperventilation, to avoid blood acidosis at the end of the apnea, and in the same time avoiding excess of the negative effects of hyperventilation (high heart-rate, brain hypoxemia, arterial vasodilatation, brain hypoxemia, depletion of myoglobin stores, ...).
The 17 minutes in Blaine's stunt are not exceptional. Already in past there were known O2 breath-holds of over 20 minutes, just they were done more for the research, and not for breaking records, or for TV shows, so nobody cared about registering them with Guinness
However, it is apparent that by experimenting and finding the optimal technique, the record can be still improved significantly. Only few people are interested about doing it, because it has no practical interest (you definitely cannot use the O2 apnea technique for normal freediving), and the experimenting bears serious risks of damaging your organism. And since healthy lungs are primordial for every freediver, only few of them take the risk to play with it. Tom Sietas is an exception - he did many O2 records in past, but doing it very safely, increasing them always just by a few seconds, and staying very far from his physiological limits. He is able of 10 minutes plain-air breath-holds, but kept the O2 apneas just around 15-16 minutes (50% above his normal apneas), although he would be certainly able doing much longer ones. I think he does is especially because the TV stunts are inportant source of revenue for him, and by increasing the records by a few secs each time, he can continue doing the shows for several years.
Seeing the data from Blaine's record, where his heart-rate was around 100 bpm almost during the entire breath-hold, and knowing that he has very good skills of relaxation and controlling his HR even under the stress of TV shows, it is almost certain that he's overdone the pre-apnea hyperventialtion, or that he used chemical substances modifying his blood pH. The resulting low CO2 level (high pH) then indeed impacts greatly the heart-rate. I hope they monitored also data about the bood pH, O2 saturation and other values, so that we learn little bit more. It looks very probable, that if the hyperventialtion was little bit lower, the heart-rate might have dropped much earlier, the total O2 consuption and CO2 production would be smaller, and likely he would be able to continue longer.
