An email to Jill, re: the “you know it’s true” statements when (trying to) discuss disagreeable topics.
tl;dr version: it’s not an argument but an actual claim to knowledge that could be legitimate, depending on how well one person knows another. A simple reminder that clams to knowledge can be legitimate through means other than logical inference. In this case, it’s through a form of empathy.
This is an interesting topic to bring up, because I don’t think it qualifies as a formal fallacy, since it doesn’t take the form of an argument. Neither is it an informal fallacy because its power doesn’t derive from the ambiguities of language.
I think it may actually be an expression of a claim to knowledge, gained through an empathetic connection. If you are in disagreement with someone who knows you well, it’s possible to make a sound claim that “you know it’s true,” if they are familiar enough with the way you reason. The claim should almost be worded: “you should know it’s true,” or, to add the unspoken part of it, “you should know it’s true, because I know the way you reason and how much weight you put on certain propositions.” To make this claim to someone online is weird, because a lot of what goes into the argument is in-person empathy; lots of non-verbal cues can be read that don’t come across with words on a screen.
In reality, the phrase is an intensifier, because the person saying this to you is doing it more out of frustration at his own inability to guide you into understanding the logic stream inside his head. Due to empathy and familiarity, he knows you would agree with him but there’s a linguistic barrier preventing effective communication.