The problem might be that whenever you are dealing with an unfolding of the unconscious mind, there might be surprises that might shock you and more when you are dealing with the astral plane. Perhaps if someone is feeling a little unstable or has depression or serious mental problems, this might not be the best practice for them unless guided by an expert and even then you may still have fear about you. It is fear that needs to be mastered.
I encountered the aspect of fear myself. I stopped practicing my particular technique for a while until I felt I could Master the fear aspect, until I could hold my thoughts calmly and steady, unwavering no matter what.
Examples…if you had an army of fearsome looking warriors coming towards you, how do you think you would respond???
If you saw deceased relatives, how would you respond??
If you saw a demon like face in front of you, what then?
If you were aware of a horrible death you once had…what then?
When Buddhists draw over and over again (in sand) ugly demons, these Demons are the sights they will encounter in the death process. They do this so that they are fully familiar with the sight of these demons. They have pictures of them, carvings etc so that when they encounter them they can calmly dismiss their presence. Any holding on to these sights with fear will alter the outcome.
Hindu temples are adorned with dancing girls…these dancing girls are the ones you will see when your desire for sex enters the stream of meditation, when the need for sex is your weakness they will appear to tempt you…(the need for sex is a powerful one in our nature so it is a common sight)…the temples have ugly demons too. Basically,they are adorned with the sights of the unconscious minds deeper parts and the astral planes sights. The most common ones everyone will encounter.
A beginner in Yoga Nidra (and not just Yoga Nidra) may be unaware of these facts. A partially trained person may be aware but may not have encountered anything too disturbing.
If you hold a strong steady calm presence , there is nothing to worry about, but you may not know how you will respond until you are in this position. This is why I took time out to master this problem.
There is a drug in South America which I cant remember its name. The Shamans take it.
Two guys went out there to try it. During the experience, one of them was confronted by a figure that was part human and part animal. It sat next to him and looked directly at him. This guy commented that he was so scared he brought himself immediately out of the experience. He said he was going back there again but was now aware that he may see things like this and could then try to hold steady to stay with the experience longer to see where it takes him. The Shamans of course are very experienced in these matters…generally, we are not.
The weird beings seen in drawings on Egyptian tombs/pyramids are of this process.
On something else…The great Edgar Cayce used this technique. The thoughts were that he was practicing self hypnosis. He/they didn’t know the actual practice was this.