This is a really weird, but really neat concept. Where hidden variable theories assert that the state of a particle is fixed at any given point in time (but tha

This is a really weird, but really neat concept. Where hidden variable theories assert that the state of a particle is fixed at any given point in time (but that quantum descriptions don't fully capture this state), retrocausality seems to say that the state of a particle is under-determined at any given point in time, and when the undetermined fragment of state is needed to determine the outcome of some future experiment, the result of the experiment adds information to the past in a consistent manner. But this has the side-effect of correlating all experiments that depend on this state! I think this also clarifies why retrocausality doesn't allow information to be transmitted back in time. Every observation on the under-determined state in the past by definition doesn't rely on the undetermined fragment, and hence is consistent with whatever choice occurs in the future. There's some flavor of monotonicity here, but it's pretty bizzare!