Historically, therapists have re-traumatised their clients rather than help them integrate the traumatic experiences they have had.
These days, most people have heard of Post Traumatic Stress Disorder (PTSD),
PTSD happens is when your brain is not able to integrate a powerfully upsetting experience. Rather than a memory that fades over time, these memories are physical and mental re-experiences of the original trauma.
During a trauma experience, our brains have to operate on the bare minimum resources to survive. Any usual functions that are not necessary for survival will disappear until we know we are safe. However, our brains find it hard to switch out of threat mode, and we may stay hyper-vigillant long after we need to be. Often, many years longer than we need.
Flashbacks
When we process our usual day-to-day experiences, we can recall them as stories with a beginning, middle and end. Flashbacks are different because they don't tell a story, as they are often a still image or series of images, that get recalled, rather than a long moving image. This is how the memory was laid down in the brain when the experience happened.
In psychotherapy, the work is to help the brain of the client integrate the traumatic experiences, without re-traumatising them.