How is it possible that flashbacks are real? ptsd?

Real, like dreams: Flashbacks are real in the same sense that dreams are real. A flashback is a "re-experiencing" symptom of ptsd. They tend to make a person feel or act as though a traumatic event is happening again. The more provocative the event, the more likely it will be that it will be vividly re-experienced in the future.

PTSD: Yes flashbacks are real psychological events. But they are not happening at that moment in time. Ptsd is a real psychiatric diagnosis and it needs professional help.

Brain physiology: During a full blown flashback, the person often believes that thery are in the situation being relived. Auditory, tactile & visual hallucinations can seem very real. Under the right situations a fight or flight reaction is triggered & old memories or fragments of memories are retrieved & become the person's here & now. These dissassociative episodes can cause great impairment & psychic pain.