well may I comment here something. Contractions of all sorts are originating in same area of our brain, be it vomiting (as if being sea sick), ejaculating or urgency to breathe. Some people who got over extreme sea sickness know the fenomena of being unable to bring their PG21 experience to its contracting end, simply because contractions of any nature have been severely inhibited while fighting off sea sickness. My guess having freediving contractions may very well be bringing someone close to another sort of contractions, naturally. I'd say this is counterproductive, like someone said - blood needed elsewhere. I tried thinking pleasant things doing tables but quickly found that thoughts shouldn't be anywhere past PG13 or I am about to BO untimely.
Peace.