It is said in the scriptures that because intermediate beings have no physical body, they tend to be very unstable and restless, unable to sit or stand still, just drifting aimlessly, like bodies in a dream. As intermediate beings long for a body of their own, many try to return to their old bodies. However, body and consciousness have separated and the old body is already a past. As the relationship between the body and consciousness has ended, there is no way the intermediate beings can ever go back to their previous bodies. At this point, the deceased who has lost that sense of belonging will feel very sad and distressed. Afterward, the deceased will enter one of the six bardo states, chönyi bardo or the luminous bardo of dharmata.
When entering chönyi bardo, many people will encounter blinding light and frightening thunder-like sound along with numerous Buddha images, mandalas and so forth. Many near-death-experience accounts in the West have also reported similar situations. However, these are mere illusions in the view of scientists. The fact is that these phenomena are not illusions at all but memories beyond the brain.
~Depicted from THE HANDBOOK'S FOR LIFE JOURNEY - On Death And Rebirth-How to Take Rebirth