The core is empathy... you have to show that you genuinely accept and make space for the other persons idea and emotions, it's the mindset behind the strategy of active-listening. If you simply use the strategy of "so you're saying that" without genuinely trying to understand the other person, it comes off as straw-manning. Similarly, if you are telling someone else what their emotions are, instead of making space for their emotion, it comes off as judgmental.

I think something else you're getting at here is that this is something that's very hard to do with a stranger over the internet - the requisite body language, rapport, voice tone etc all get lost (and therefore more useful in most real-world cases of trying to change someone's mind)

Edit: one final thought on this is that active listening isn't much used in debate. It's much more useful for dialouge and rapport building than trying to convince someone with facts.