Stef,
Great question!
First of all, I think it's important to recognize that this is one of the more advanced and complicated freediving skills to learn. I learned the mouthfill from Eric in 2000 and I still find that every year after a long break of a month or two that I have lost the ability to do it perfectly. I think I know it by heart, but when I unexpectedly run out of air at a shallow depth, then I know I've let my concentration waver.
How to practice the mouthfill for deep dives?
Do it as often as you can. If you dive recreationally on FRC, then you will be doing it a lot since it makes diving past 20m much more comfortable if you use the mouthfill. If you are an inhale diver, then I would advocate using "circular equalizing" (a term coined by Tylerz). That is use a mouthfill from the surface, then when it runs out, do another one, so that you always have air in your mouth and you are always having to close the epiglottis and open the soft palate. If you want to do a deep dive, you could do your first mouthfill at 5m and then do a second one at 25-30m, which should last you to the bottom.
Only by practicing it as much as possible will you get better. Not every mouthfill is perfect, but the more you do them, the more you'll get a sense of what makes a perfect one.
You could also do the mouthfill practice with focal points, things you concentrate on to improve different aspects of the action.
1. Focus only on the jaw thrust and widening your eustachian tubes
2. Focus only on head and body position for driving air into your cheeks
3. Focus only on fillling the cheeks until they are bursting
4. focus only on returning your head to the neutral position and keeping your epiglottis closed
5. Focus only on staying as relaxed as possible while doing all of the above,
etc, etc...
You can develop your own focal points based on an aspect of the technique that gives you trouble.
My favourite dry land decoupling exercise is simply to alternate breaths through the mouth nose so I can isloate the soft palate muscles, then I stop and close the epiglottis and continue opening and closing the soft palate so that it makes a clicking noise.
Pete
