Surely it means: a god steps into the world beside you, and offers you the choice: "either take the A-or-C gamble, in which case I'll guarantee to arrange for A or C; or else refuse it, in which case I'll go away and leave your life uninterfered with". Or, in other words, B means whatever future or lottery-between-futures you'd have been facing if you hadn't been given this offer.
My problem with this definition of PLP is that it does weird things with determinism vs. free will (just like some people complain about Newcomb's problem statement).
If I win A will I live to a healthy 120 no matter what I do, like quantum immortality? Could I, e.g., infect myself with AIDS, drink a deliberately randomized cocktail, and have it turn into a magic AIDS cure because I'm guaranteed health? Could I cause an atom bomb to malfunction by standing next to it and setting it off? Could I donate organs and limbs for transplants and then regrow them? ...
... Oh, no! I've been shot!
— C3PO
A strange sort of paralysis can occur when risk-averse people (like me) decide that we're going to play it safe. We imagine the worst thing that could happen if we go ahead with our slightly risky plan, and this stops us from carrying it out.
One possible way of overcoming such paralysis is to remind yourself just how much danger you're actually in.
Humanity could be mutilated by nuclear war, biotechnology disasters, societal meltdown, environmental collapse, oppressive governments, disagreeable AI, or other horrors. On an individual level, anybody's life could turn sour for more mundane reasons, from disease to bereavement to divorce to unemployment to depression. The terrifying scenarios depend on your values, and differ from person to person. Those here who hope to live forever may die of old age, and then cryonics turns out not to work.
There must be some number X which is the probability of Really Bad Things happening to you. X is probably not a tiny figure, but instead significantly above zero, which encourages you to go ahead with whatever slightly risky plan you were contemplating, as long as it only nudges X upwards a little.
Admittedly, this tactic seems like a cheap hack that relies on an error in human reasoning - is nudging your danger level from .2 to .201 actually more acceptable than nudging it from 0 to .001? Perhaps not. Needless to say, a real rationalist ought to ignore all this and take the action with the highest expected value.