What is the eligibility criterion for practicing Kumbhak?

Question: In the Sixth Chapter of Geeta, Dhyaan Yog, Lord Krishna said Concentrate between the two eyebrows and stop the prana vayu and apaan vayu. I tried to meditate this way. I started practicing kumbhak slowly. Now I do kumbhak for 3-5 minutes each time. When I sit to do kumbhak, I lift my apaan vayu and retain it for 3-5 minutes, in this situation, my body temperature rises within 2 minutes and after 4 minutes I see an open space like a black ocean within myself and I keep on going deeper and deeper into it. All this happens on its own when I do kumbhak for 4 minutes and I feel afraid at this point. I dont know what to do further. Please guide me. GURUMAA: Munish, its true that Krishna has talked about concentrating between the eyebrows but I have a huge question about your interpretation about the stoppage of the prana and the apaan vayu. Where have you got this understanding from? Well, regarding Kumbhak I can tell you that you cannot afford to be in a rush with Kumbhak. Kumbhak cannot be practiced by a body which has not been purified. The body becomes eligible for doing kumbhak only after having practiced Yoga Asana for at least 2 years along with the practice of pranayama like anulom-vilom, kapalbhati, bhastrika etc. Having followed the regime of Yoga Asana and pranayama, holistically you can start up with 2-4 minutes of Kumbhak (retention of breath). The retention of breath must be learnt from a right teacher. You cannot play with prana, its like a snake which can kill you. Any practice, which is not done with right understanding can be dangerous. Thus, I suggest that dont do kumbhak on your own. Practice Yoga Asanas like Surya Namaskar, Ushtrasana, Mandookasana, Janu Shirshasana, Ardha Matsyendrasana, Sarvaangasana, Shirshasana etc. This is a basic catalogue you need to follow along with a satvik, pure vegetarian diet coupled with disciplined sleeping time, wake up time. Also, you must do pranayama for at least 1 hour everyday. After following this protocol for a year or so, your body will be able to take the whole pressure and energy generated by doing kumbhaka. Visit http://www.gurumaa.com for more videos