Kodama, personally, I have only managed to do handsfree up to possibly 8m head down using free immersion. I believe the ability to do it is linked to mask equalization. The best resource on BTV/VTO on the web, in my humble opinion, is Santiago's post at
https://forums.deeperblue.com/threa...ization-partial-btv-that-worked-for-me.99013/ where he describes using the “pressure from the mask” (point 4). My personal theory is that hands free equalization, of the type you and me do, opening and closing, rather than constantly opening the Eustachian tubes, is dependent on creating a region of high pressure in the mask:
Point four about "pressure from the mask". What actually happens physically is that when under pressure both the air in the middle ear and the air in the mask connected to nasal passages contracts. What we want is for air from the nose to travel to the middle ear. So we need to create a higher pressure. We are aided in this process by being able to control the amount of air in the mask. And this is done by Santiago by pressing down the mask to his face to compact the air molecules or by equalizing the mask such that there are more air molecules packed into that fixed space. Once this pressure gradient is achieved where the mask space connected to nasal passage space has a higher pressure than the middle ear, the Eustachian tubes are then opened, to allow for equilibrium to be achieved.
This also suggests that there must be a contained space - meaning either the soft palate or the glottis is closed prior to the opening of the Eustachian tubes. So the pressure can build up and does not escape into the lungs. Which one is closed? Ergonomically, it is more efficient for the soft palate to be closed because it's easier to build up the pressure in a smaller space. Also, it's the soft palate that's more involved in hands-free than the glottis.
What do you think? Maybe you can play around with the mask and see if it helps? I practise hands free mostly by holding my nose and sucking to create the pressure difference and then unblocking the ears. A new method I tried is going to the deep end of the pool on full exhale and then equalizing (not the most comfortable experience!).