Physical force itself is not an argument, obviously. But consider an exchange like this:
- Person A: "I believe X, and I think you should agree with me."
- Person B: "What reason can you give for agreeing with you? X seems ridiculous..."
- Person A: "If you don't agree with me, I'll kill you."
Here the act of violence is offered up as a reason to produce agreement. The idea of lethal force is used as a substantive argument in the debate; whether or not that idea is put into practice is more or less irrelevant.
In rational argument, people expect that substantive claims will address the ideas under discussion. The power of a rational argument lies in its ability to cause someone to revise their worldview solely on the strengths and weaknesses of the concepts themselves: their consistency, their logical validity, their appropriateness and applicability... It is generally considered to be bad form to import other forms of power — physical violence, social authority, shame and guilt, money, factional norms, etc — into the discussion. People do it all the time, of course, but it is bad form. Most of the so-called 'informal fallacies' are not really fallacies in the proper sense, but are power-plays of this sort, and ought to be understood on those grounds.