Yoga Siddha Body Development

What exactly is a yoga siddha form? What are the characteristics and qualities?

A yoga siddha form is a subtle body which is resistant to physical rebirth and which has an innate tendency to complete the austerities which are necessary for liberation or for translation to a spiritual dimension.

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Where is its destination?

The destination of a yoga siddha form is the astral realm which is above the Swargaloka heavenly places but in terms of association siddhaloka is an astral territory where great yogins have reserved subtle realms for their advanced disciples who have resistance to rebirth and strong tendency to keep practising the yoga austerities to proficiency.

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After you acquire one, does it ever expire or revert to the previous lesser qualities?

A yoga siddha form can revert back if the yogi is careless or if he is sent down to teach on lower planes of existence. Careless means if he somehow or the other hears or perceives in any way people who are in the lower astral regions or on the physical level of existence. If he fails to keep his attention upwards he may become aware of the condition of people in lower astral or physical place, and that awareness could easily cause descent.