Here's an example from Fischer's talk-- a "Frankfurt case," since Harry Frankfurt discussed a case like this in his 1969 paper "Alternative Possibilities and Moral Responsibility."
Because Black dares to hope that the Democrats finally have a good chance of winning the White House, the benevolent but elderly neurosurgeon, Black, has come out of retirement to participate in yet another philosophical example. (After all, what would these thought-experiments be without the venerable eminence gris—or should it be noir?) He has secretly inserted a chip in Jones’s brain which enables Black to monitor and control Jones’s activities. Black can exercise this control through a sophisticated computer that he has programmed so that, among other things, it monitors Jones’s voting behavior. If Jones were to showany inclination to vote for McCain (or, let us say, anyone other than Obama), then the computer, through the chip in Jones’s brain, would intervene to assure that he actually decides to vote for Obama and does so vote. But if Jones decides on his own to vote for Obama (as Black, the old progressive would prefer), the computer does nothing but continue to monitor—without affecting—the goings-on in Jones’s head.In fact Jones (shown in white in the picture) shows no inclination to vote for anyone but Obama, Black doesn't intervene, and Jones does vote for Obama.
What's the point? Well, it's often assumed that determinism rules out moral responsibility. Why? Because if determinism is true, then whatever you do, you couldn't have done otherwise; whatever you choose, you couldn't have chosen otherwise. On the face of it, it seems as if a person who couldn't have done or chosen otherwise isn't responsible for what he did do or choose.
But perhaps not! Jones couldn't have done otherwise than voting for Obama. But is it really obvious that he's not responsible for doing so? Black never actually intervenes (he's a "counterfactual intervener"). All unfolds exactly as it would have, had Black not inserted the chip. Fischer makes a cautious assessment: if Jones is not responsible, it's not because he couldn't have done otherwise. That then calls into question the standard reasoning that runs "determinism...so couldn't have done otherwise...so no responsibility."
Once you get to thinking about Black and Jones, you just can't stop. Have fun.