Whenever you are suprized, face immediate danger, or encounter something frightening, make a self control check. If you fail, you can do nothing except attempt to flee at top speed directly away from the danger. You may take any available active defenses while fleeing (but you may not ready an unready weapon to parry with) and you may detour around minor obstacles. If you are not able to run away (due to restraints, being locked in a room with the danger, or some such) you struggle uselessly or batter yourself against the walls in your attempt to escape. You can make a self control roll to stop your flight, this is at +3 once there are no longer any apparent sources of danger.
This disadvantage has one side effect - if you are mentally stunned, you flee as above rather than doing nothing for as long as the mental stun lasts if the mental stun is a result of a failed fright check, combat situation, or suprize. The same is true if you Freeze due to suprize in a potential combat situation - you run for the duration instead. If taken as a -1 point quirk, this side effect still applies.