I think this is some kind of supernatural "free will", and no mechanism has been found for such a thing in science. It's associated with mind–body duality, souls, homunculi inside the body, that kind of thing.
It doesn't have to be super natural either, though. By the nature of it's existence, the universe proves that there can be chains of events spawned that completely undermine our current treatment of causality. A realistic model must include at least one of the following:
1) Events with no cause such as the beginning of the universe or broader multiverse, depending on your beliefs or 2) Infinite regressions of causal chains if you believe that the universe or God (or a god) has always existed or
3) Some combination of the above.
In all of these, somehow there can be uncaused effects, but in 1) and 3) some causal chains can just begin existing, like free will would require.
1) Events with no cause such as the beginning of the universe or broader multiverse, depending on your beliefs or 2) Infinite regressions of causal chains if you believe that the universe or God (or a god) has always existed or 3) Some combination of the above.
In all of these, somehow there can be uncaused effects, but in 1) and 3) some causal chains can just begin existing, like free will would require.