Juha,
You can go to 15m with a 'mouthful', with full or empty lungs, as long as you keep your epiglottis closed. With full lungs, you can 'cheat' and equalize the normal way. But, with empty lungs, you must be VERY careful! 15m with empty lungs in an enormous stress on your chest, and you could easily black out as well! So at first, practice with full lungs, or half a lungful.
Eric Fattah
BC, Canada
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