When a disputant insists on introducing irrelevant considerations, ignoring his opponent's logic and evidence. He cannot grasp the whole of the issue--or the principle underlying it--so he focuses on some small part (usually just one word) and directs his rebuttal to an attack on that tiny bit which is all he can perceive. He views things through his specialized eyes, extracts a part of the truth and refuses to see more, sometimes quoting your least significant statements, in order to make it appear that you have said nothing better. When something is too strange or complicated to deal with directly or comprehensively, he extracts whatever parts of its behavior he can comprehend and represents them by familiar symbols--or the names of familiar things which he thinks do similar things.
"What do you mean by ------?" Where ------ is any word included in your presentation, usually a quite ordinary word which your opponent uses without any difficulty in other contexts. (This is so prevalent that perhaps it should be given its own name: The Dictionary Fallacy.)
Some Ad Hominem arguments probably have the same source: He can't see your ideas so he directs his rebuttal at your person. Or will simply start talking about something he CAN understand--the result being a jarring change-of-subject in the discussion.
He seizes upon one instance and constructs a generalization from it: Observing that I don't like clams, he concludes that I have an aversion to sea food in general. She sees something happen once or twice and concludes that it is a regularly-occuring phenomenon.
These responses are not consciously deliberated, but result from his inability to perceive the focal idea of the discussion. His only alternative to one of these responses would be bovine immobility--unless he possessed a sufficient degree of intellectual acumen to realize his lack of comprehension, and a sufficient degree of self-esteem to admit to it.