One of the biggest challenges in working with survivors of sexual trauma is the client's sense of failure if, during the course of the assault or rape, s/he froze and did not fight or run. The client feels s/he failed to do what s/he "should" have done, or could have done, to fight off or run from the perpetrator.
Of course, we know this is not true. And we might try to help the client understand that any response that kept him/her alive is the right response. Often these attempts to reframe the situation fall on deaf ears.
However, I have found that offering a biological and evolutionary explanation seems to carry a little more weight. Below is one of the best explanations of the neurobiology of the freeze response, courtesy of Joseph LeDoux.
"[F]reezing is a beneficial response when faced with a predator. Predators, primal danger for most animals, respond to and are excited by movement. Keeping still in the face of danger is often the best thing for the prey to do. Because millions of years ago animals who did so were more likely to survive, today it's what most animals do, at least as an initial line of defense. Freezing is not a choice but an automatic response, a preprogrammed way of dealing with danger."
"What's interesting is that freezing also occurs if a rat (or other animal) clearly hears a sound that preceded an aversive stimulus (a mild electrical shock of its feet) on some prior occasion. There's no predator around in this case, so how is the connection formed? The sound is a warning signal. Any rat that survives an encounter with a cat or other predator should store in its brain as much about the situation as possible so that the next time the sounds, sights, or smells that preceded the arrival of the cat occur, those stimuli can be attended to in order to increase its chances of staying alive."
~ Joseph LeDoux, Synaptic Self: How Our Brains Become Who We Are, 2002, p. 6