A is walking down the street minding their own business carrying a purse. B wants what's in the purse but is afraid that if B tries to snatch the purse, A might cause trouble for B (such as by scratching and kicking B and calling for help). It is implicit in this situation that if B does not bother A, then, in trade, A will not cause trouble for B.

B realizes, too late, that B should have worn something really scary to signal to A that B was committed to being bad, very bad, so that neither kicking or scratching nor calling for help would be likely to be of any use to A. This would have strengthened B's bargaining position.

A, not being an idiot, is aware of this as general fact about people, including about B, and breathes a sigh of relief that there aren't any scary-looking people in sight.

Is A an extortionist? Is A continually extorting good behavior from everyone around A, by being the sort of person who would kick and scratch and call for help if somebody tried to snatch A's purse, provided that the purse snatcher had not effectively signalled a pre-commitment to snatch the purse regardless of A's response? A is implicitly extending an offer to everyone, "don't try to take my purse and, in trade, I won't kick and scratch and call for help." A purse snatcher who effectively signals a pre-commitment to reject that offer (and thus to take the purse despite kicking and scratching and calling for help) places themselves in a stronger position in the implicit negotiation.

This seems to follow all the rules of the offered definition of extortion, i.e.:

How about "Extortion: Any offer of trade (t) by A to B, where A knows that the likely utility of B would be maximized if A had in advance treated (t) as certainly rejected."

In short extortion is any offer to you in which you could rationally wish you had clearly precommitted to reject it (and signalled such precommitment effectively), and A knows that.