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Well, I am a little bit confused at the moment. To me, it very much feels that during the breath control sequence you draw energy up from the base of the torso (sexual energy) take it up to the head where it gets changed into spiritual energy. Then due to the partial out breaths this now spiritual energy is released into various parts of the body.
I would also echo what Earl said about the non-standard mechanism of Flying Phoenix. There also is a very subtle aspect in it that later allows you to access a particular type of heavenly energy, so it's not ever only about the energy of the physical body.
The thing is, having read what Terry Dunn has written, he says that although there are Back Fu Pai practices that use sexual energy, Flying Phoenix doesn't, so I'm not certain on this one.
The vital energy of living beings is fundamentally neutral and unimprinted. What could be described as the sexual charge is nothing else than the body-mind exhibiting one particular karmic conditioning: sexual maturity. Hence, not having even the subtlest thoughts about sex leads to spontaneous celibacy, which is one of the foundations of advanced yogic practice.
What is the use of sexual energy anyway in yogic arts? It can't be the conscious stimulation of sexual thoughts obviously because this would lead to the stimulation of reproductive functions (sex hormones and fluids), but there would have to be some kind energetic transmutation that briefly touches upon some sexual centers of the body, such as the navel chakra region, and giving a feeling similar to sexual bliss.
Young children have plenty of vitality and free spirit to go with it, but they haven't accessed their innate sexual impressions that trigger puberty and sexual desire. This is why their energy goes towards growth and their desires typically follow more innocent concerns that don't end up with forming great karmic entanglements which sex is pretty much all about. Could they safely practice internal arts that specifically utilize vital energy in a manner that goes through the sexual centers of the body? I don't know, but it could provoke unwanted consequences such as early puberty if not done entirely correctly.
In general, I would advice to be very careful with the topic of "sexual energy" because there are a lot of misconceptions and false advertisement associated with it. Almost everything marketed as "sexual practice" are ploys to spiritualize common greed and lewdness.
My curiosity has gotten the better of me, I saw 5 DVDs of Golden Flying Phoenix being demonstrated by GM DW and I really wanted to see how he performed the meditations. Having just recently ordered all of Terry Dunn's DVDs, I have more than enough practices to keep me busy for a couple of years, however it can still be really useful to see how other experts perform similar movements. I have no intention of studying them, but I am looking forward to watching them.
Golden Flying Phoenix is very advanced and has a specific yogic purpose which needs a particular type of readiness or supplementary instructions.
Beware that there are a lot of ill-gained and immorally peddled Bak Fu Pai videos and DVDs in circulation.
In my opinion, the level of practice and the subtle nuances which GMDW so casually demonstrates in all his videos, even those in Youtube, is inaccessible except to very high level yogis. Guard yourself against misimpressions and forming opinions in case you decide to follow through and watch his demonstrations.
In the beginning, sound will be very disrupting to the practice, and this is because your channels are opening and you're more sensitive. Later on, as you balance more from this proper practice, you won't notice disruptive sounds at all. I couldn't handle even a train passing by or neighbors closing and opening their doors for the first few months. I was extremely irritable then, and after a while, it wasn't that I got used to sudden noises, they just didn't register, as though the FP qi is filtering for me rather than me growing accustomed to it.
Give yourself some time and some music can help, especially Buddhist mantras.
I'll add a warning that while the more advanced trance states have a bigger threshold for disturbances, the actual physical and energetic shock for suddenly getting startled is going to be more severe:
How to Arouse Someone from the State of Samadhi As a rule you should not touch them or shake people who are in samadhi because this can hurt the individual. This is because when someone is in samadhi, his or her body becomes extremely soft and vulnerable. For instance, there is a famous story...
There's nothing to worry about for the beginners or even many of the seasoned yogis, but it's good to have a balanced and realistic impression about what the disruption of the practice can cause. I know one yogi who had accessed the a mild dhyana trance for the first time when his doorbell suddenly and loudly rang, which shocked him back to the ordinary consciousness. He described that the bliss of trance turned into a sharp pain, so he took more precautions after that.