In this video, Stan Grof talks about how shamans have used breath control to create non-ordinary states of reality. Grof developed holotropic breathwork when LSD became illegal, as a way to generate the same or similar states of consciousness.
I've used holotropic breathing in my own shamanic work, combined with Michael Harner's shamanic drumming CD. I've found this to be an incredibly useful way of entering inner space, specifically the unconscious mind. Having also used all the standard hallucinogens, I can say it bears no resemblance to any of them (although a very small dose of LSD is most similar in that consciousness shifts more easily -- without the hallucinations of higher doses).
The process I have used involves holotropic breathwork until I sense my consciousness opening up (the drumming CD is playing already during this process). I then visualize an opening into the earth, for me it's small cave in the Siskiyou Mountains of Southern Oregon that I explored on a couple of occasions.
When I enter the opening, the scene quickly shifts to some sort of cosmic spiral tunnel through which I move very quickly. When I come out the end I am almost always in the same landscape (going through a different opening takes me to a different landscape -- I find the consistency in this aspect of the work very interesting).
I generally go in with some sort of question, which dictates the experience that follows. When the CD signals time to emerge, I make my way back to the opening through which I entered and move back up the spiral tunnel.
Only once has this pattern varied. The last time I journeyed, I ascended into the heavens during the experience -- more precisely, I was summoned by some kind of archetypal female energy. I didn't return from that experience in the same way, but I did emerge from the originating cave when the journey was over.
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Anyway, I promised someone I would blog about this work a while back and hadn't done so until now. I've hesitated because shamanic work has so many negative New Age connotations to a lot of people.
Shamanic work can be integrally informed, however, and need not be some New Age load of regressive pre-trans fallacies. If one understands the process in terms of psychological processes more than as some literal journey to the underworld or whatever, we can avoid the pitfalls in the way shamanism has been co-opted by the New Agers.
Still, it helps to suspend disbelief while engaging in the process of the work. If we are in our rational minds, we will not be able to surrender to the experience. I find it best to just be open, in as much of an observing space as possible, much like in meditation.