From what I've seen, in context, a whataboutism is concerned about the argument of moral authority and precedent from the person accusing a AES or progressive of something bad...
Here's some rules:
Apply it when it relates to the precedent of incidents that seem exceptionally bad, when committed by Actually-Existing-Socialist countries, but are ignored, if not justified in Capitalist ones... they could be explained as historical thinking of the time or justified under circumstances
For example: https://hexbear.net/post/2329752 (Homosexuality laws compared between historical Soviet Union and the West, lemmygrad meme)
Show
However, don't use irrelevant ones to detract from past mistakes, they make you look daft and aloof
I'll explain more later on...
You will never ever own a lib by catching them saying something hypocritical. They don't care. They care a lot about hypocrisy when a republican does it but not when they do it. They have their alternative facts that they have decided are 'reality' (like Russia using shovels to fight or whatever) even if real reality flies in the face of that. These facts shield them from hypocrisy.
Also arguing with libs online is an enormous waste of time, just find a better hobby.
Case and point, they inveted the word whataboutism to redefine "obvious hypocrisy" so that pointing out their shameless hypocrisy doesn't count because they're the ones keeping score.
The internet is an enormous radicalizing tool. Posting is the agitprop of the 21st century.
I know, it's just that I just scroll down posts and end up looking at online arguments involving that all the above...
it's not about the lib you're talking to directly, it's about the ones reading the conversation who are less on the defense against your points. Literally right now I don't expect to change your mind on the topic but rather to encourage and reassure other people who will see this.