I think that perhaps one of the reasons why chunner's may leave the arm out is because that's how most chunner's get their inputs--by feel.
They expect the punch to hit it's mark, if it is intercepted, that interception becomes the catalyst for another movement. It's the second trigger so to speak. If the punch is left out (which I HIGHLY recommend not doing) perhaps is it because the person "expects" the person to react just afterwards and they maybe expect that to be the next trigger.
Not sure...leaving an arm out is bad joo joo either way.
"I don't know if anyone is known with the art of "sitting on your couch" here, but in my eyes it is also to be a martial art.
It is the art of avoiding dangerous situations. It helps you to avoid a dangerous situation by not actually being there. So lets say there is a dangerous situation going on somewhere other than your couch. You are safely seated on your couch so you have in a nutshell "difused" the situation."