Throughout the centuries, people have used different techniques to dive deeper and longer. Whereas our ancestors used their lungs to explore the underwater treasures, using air filled bladder from animals and other artifacts announced the venue of our modern scuba tanks.
Submit to laws of physics, a body immersed has to cope with the increasing ambient pressure (+1 kg/cm2 every 10 meters). At great depth, it is impossible to counter the pressure on the air cavities by using air from collapsed lungs.
Through the years, breath-hold divers improved their body control (eg. diaphragm, lung packing), fine tuned their equalization techniques (Vasalva, Frenzel, ...), and learned to manage and to store air outside their lungs (eg. stomach, mouth). But counting on an air volume doomed to disappear (no matter what artifact used), unless you go for the foolish idea of bursting you eardrums, depth can only be stretched up to a certain extent = limit.
The ACF, a water- instead of air- based technique, is the outcome of a reflexion on the human being lifting, once and for all, the human barrier (when it comes to pressure constraints on the air cavities, of course). Don't worry, I still have tons of other things to solve/understand to sustain my progression. The principle: liquid are incompressible, meaning that if my ears and sinuses are filled with liquid (water) they will not be submitted to pressure any more. Simple, right?
Passive Flooding of paranasal sinuses and middle ears as a method of equalization in extreme breath-hold diving:
'....Our subject is a 36-year-old physical therapist, who started breath-hold diving (BHD) at the age of 28. During the first 3 years, he used the conventional BHD techniques of equalising rigid air spaces by repeated Valsalva manoeuvres. Then, finding that these techniques were possibly traumatic and required active muscle contraction, he started training in passive equalisation techniques. This involves keeping the nostrils open during the descent, and allowing water to passively enter the nasal cavity into the maxillary sinus ostia and Eustachian tubes as the air volume decreases with depth.
Over the years, he perfected this technique, and now it has become a quite natural BHD behaviour for him, inasmuch as he finds difficult to switch back to the conventional techniques.
Using this technique, he attained in June 2005, in the course of a few days with increasingly deep BH dives, a new record depth of 209m, 38 m deeper than the previous record. Using MRI, we obtained images of his sinuses and middle ear cavities before (figure 1,2) and after (figures 3,4) instilling water through the nostrils.
To do this, he simply used tap water, from a plastic bottle, poured sequentially into each nostril. By rotating the head in various positions, our subject was able to fill most of his sinuses and middle ears with water, without any noticeable reflex reactions (sneezing, coughing), pain or discomfort.
On the MRI images, water can be clearly seen, not only in the maxillary, ethmoid and sphenoid sinuses, but also in the middle-ear cavity and in the mastoid recess. After the imaging, only part of the water was passively evacuated by raising the head in an upright position, the rest was evacuated progressively over the next few hours...'
It goes without saying: DON'T TRY THIS at home! FYI, the article also outlined the potential risks of such technique applied to human.