The critical element of mansplaining (the reason the tongue-in-cheek name stuck is because every woman has experienced it and it's so hilariously dead-on, if you've ever been on the receiving end) is the assumption, from out of the blue, that the person being explained to has no pre-existing knowledge of the subject even when it can reasonably be assumed that they know quite a bit; sort of like my friend explaining my cameras to me. MY cameras. Which I own, and use.
My favorite recent example, brought to my attention by the illustrious Alty, was a guy in line to see Deadpool explaining Deadpool's history to Gail Simone
as she was also in line to see the movie. Because she is a lady, and this guy assumes that a lady in line to see Deadpool must not know anything about the character because that makes perfect sense.
Alty had to explain who Gail Simone was, because I am not a comics person, and then I LOLed and LOLed.
So an important element that I would love to get at is not only where the urge to share your knowledge comes from, but also from whence arises the assumption that the person on the receiving end, usually a woman, has no knowledge themselves.