basically be summed up as “Japanese drawn porn”. “Ahegao”, in it, can be considered a drawing genre in which the character (mainly female) is showing, during sexual intercourse, an intense amount of pleasure translated by her facial expression.
Indeed, the Ahegao slang, on Japanese web, isn’t firmly bound to what would become an exploitable face, but represents any picture, often explicit and graphic, where that kind of orgasmic face expression can be seen.