Okay, returning to the OP, if we take "mansplaining" to be using one's privileged position as a member of the class (men) socially and culturally expected to know more about the area under discussion (IT, or engineering, say) giving a patronising explanation to a woman about something she actually knows a great deal about (IT or engineering, say), what would "femsplaining" look like?
Presumably, we'd find an area (cookery?) where women were culturally expected to know more (in a domestic setting - this will turn out to be relevant), and have a woman patronisingly explain something to the men in the room when in fact they were very good cooks. (I have seen this happen in fact at a friend's pampered chef party).
But would it really be "femsplaining" in the sense that it carried the same weight in undermining and minimising the importance of the men in the room? No, for two reasons.
Firstly, the areas where women are culturally granted the small concession of "knowing more" are generally clearly delimited and set aside by mainstream culture as "domestic" or "trivial". All that happened at the pampered chef party was that the woman giving the presentation looked a bit of a twit. Compare that to the situation of a woman being "mansplained" to at, say, a large IT conference. If the audience doesn't have independent reason to know that she's actually an expert, the bloke doing the mansplaining gets to look like the big, well-informed man, and the woman is assumed (by the audience) to be the twit.
Secondly, returning to the cookery example - women's "dominance" is confined to domestic cookery - ask yourself what percentage of chefs in top restaurants are female.
So "femsplaining" is like "misandry" - one of these false symmetries which in virtue of the gender inequalities in society at large is actually, on closer examination, an entirely false word made up to silence uppity women and stop them talking about a real phenomenon - "mansplaining" - which is used to belittle and demean them in professional and political settings.