In this scenario, I suppose it would be, "a very strong guy who has a very strong grip and an invisible force field around himself that allows him to grab onto someone else and simply stand there without anything happening to him." Ultimately, any of these "does it REALLY work" questions can be answered with the circus ponies: if the questioner has the right to set up an absurd proposition as the premise for the question, why should the respondent not have the same leeway?
It only works if the questioner agrees that the premise is absurd. Pull out the circus ponies in response to a scenario that the questioner thinks is plausible, and you and your art instantly lose all credibility.
In the case of the very strong guy with the very strong grip, my response would be to demonstrate that he *doesn't* have an invisible force field, and so there are any number of circus pony-free ways to deal with him.
When someone walks into the dojo, you don't know their history. They could be an abused spouse. They could be a former ghetto kid who used to get into street fights. They might have absolutely no experience with real violence, or quite a lot. Treat their questions with respect, and maybe they'll stick around long enough for you to learn from each other.