The idea that we don't have free will. Under multiverse thoery we have no free will becuase there is a bunch of other people doing what you didnt. So your action is the only action left not taken. However in the discussed situation you are free to do what you wish but there are consequences to every action or in action. How fair those choices are though depend. You are basically asking Jack to let himself die which is not fair to ask of him. So the only fair option is do what the person with the gun makes you do.