There are several processes in your body. Some are conscious eg you moving your hand. Many more are unconscious...food being digested, or heart beating.
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And if you ignore it goes on in the background. Moreover breath employs both the prana and apana vayus moving up and down.
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Focus on breath takes the practitioner towards controlling unconscious processes. Its like using a small thread to pull a slightly bigger and bigger thread till you reach the actual rope and pull that.
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Accessing the unconscious allows you control over physiological processes but also ultimately allows access to your unconscious mind. You gain a sense of self awareness where you are able to see your actions, thoughts and even emotions in the context of cause and effect.
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You become free of being controlled by the vicissitudes of your mind. You are no angry or insecure or sensitive... You are merely the effect of a cause. This allows you to change yourself the way you like.
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