However, on the internet, specifically Gelbooru and other hentai picture sites, it's caught on as a meme*. It's used by members who are presumably masturbating to said pictures, and since comments can't be posted with fewer than three to four characters, they just either post "keystrokes" in all caps or many times.
It's used similarly to "fap" in this regard, which is the onomatopoeia for masturbation.
*A meme is a fad almost always exclusive to the internet that very quickly grows insanely popular.
Anon2- Keystrokes keystrokes keystrokes