Trance postures aid in the art of inducing trance and this is most likely due to the amount of stress on the body and/or change in blood flow to the different parts of the body. For example, there is a posture called the "Nupe Mallam" that I have tried and had rather quickly gone into a light trance. The reason for only going into a light trance was because I was very aware of my surroundings and only wanted to try the posture. The posture is rather stressful on the arm and blood flow to the leg is cut off a bit. One will experience a bit of numbness in both extremities.
Nupe Mallam diviner (Mair 1969: 94)
Here is an excerpt by Howard G. Charing on how to position the body into a Nupe Mallam posture:
Sit on the floor, leaning toward your left and supported by your left arm. Hold your left arm rigid, with your hand at a right angle to your body. Place your left hand at a spot three to five inches to the left of your body and just behind a straight line drawn along the back of your buttocks. Bend both legs at the knees with both feet pointing to the right, positioned so that your left foot is resting just to the left of your right knee. Place your right hand on your lower left leg, where the muscle indents about halfway down your calf. Move your head slightly to the left, so you are looking over your left knee, and close your eyes.
And he gives the basic tips that one should have a a clear intent and listen to drumming and/or rattling to enhance and aid in the experience, however one should know to have an intent before participating in hedge-riding. Why would one cross the boundaries for no reason? Charing's article is exploring shamanism, though, not a hedgewitch's practice.
Gore's and Goodman's writings claim that different postures have different feeling associated with them, and once again this is most likely due to the postures either restricting blood flow or being positioned in such an abnormal way that the blood flows differently throughout the body. Goodman's experimentation results on different postures and shamanic trance has in fact shown the different feelings associated with different postures and trance induction. These feelings include physical and emotional changes, such as tingling or numbness in the body and significant changes in mood.
We see different ecstatic body postures in different cultures and they have no doubt affected consciousness in some way or another because of the physiological changes occurring when one changes their body posture and ultimately changing blood flow.
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References:
☆ Barendsen, Kristin. "Sing the Body Ecstatic!" Yoga Journal 133 (1997): 166. Print.
☆ Goodman, F. D. "Body Posture and the Religious Altered State of Consciousness: An Experimental Investigation." Journal of Humanistic Psychology 26.3 (1986): 81-118. Print.
☆ Goodman, Felicitas D. Where the Spirits Ride the Wind: Trance Journeys and Other Ecstatic Experiences. Bloomington: Indiana UP, 1990. Print.
☆ Gore, Belinda. Ecstatic Body Postures: an Alternate Reality Workbook. Santa Fe, NM: Bear &, 1995. Print.
Resources:
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Where the Spirits Ride the Wind: Trance Journeys and Other Ecstatic Experiences