Early in life the child's primitive mind creates the idea of danger and masters the overwhelming feelings associated with it, learning to "tame" these feelings. Later learning to distinguish real danger from not and develop protective mechanisms.
The originating source of discomfort of all trauma lies in the helplessness of the human infant when they are flooded by excitation beyond their capacity to master until achieving greater mental development. During later life trauma, a similar uncontrollable overwhelming anxiety, felt as something terrible, floods the helpless person during the event or even after upon realizing the peril it presented and what might have happened.
When the danger experience was sudden, a later temporary psychological regression to the state of helpless infancy and destabilizing distress can occur. Understanding what is happening and telling oneself that they are now a competent adult and not a helpless infant can comfort as the mental process unwinds, healing occurs, and the person recovers their usual equilibrium.