Even if you imagine a threat, your brain will still send a fear signal from the cortex to the amygdala. The amygdala is the fear processing center of the brain, and once it is activated, it sets off the fight or flight response. So imagined and real threats can stimulate the fight or flight response.
Knowing that imagined stimuli can affect the amygdala, then you can also calm the amygdala by imagining relaxing and calming scenes, such as done with visual imagery. So imagining stressful scenes can stimulate the fight or flight response, while imagining calming scenes can extinguish it.