The ahegao, the exaggerated depiction of tongue lolling female orgasmic ecstasy which has in the the last 10 years become a staple of certain genres of extreme ero-manga, eroge and ero-anime, has its evolution charted in the below diagram:
For those not familiar with this particular line of fetishistic material, the term “ahe-gao” (アヘ顔) is thought to derive from the sound of uncontrollable moaning (“ahe,” etc) and “kao,” meaning face.
Flushed cheeks, tears of ecstasy with eyes rolled back and a tongue flopping around out of an open and drooling mouth are the standard mode of depiction. Typically the unwilling or semi-willing participant will have given way to pleasure and be either unable to resist or desperate for more.
Depictions are usually exaggerated intentionally or unintentionally to comic or at least surreal effect.
Asagi has even been adapted into a cosplay AV with authentic 3D ahegao action, such as it is:
The ahegao is not to be confused with the common ikigao (“coming face”) which is generally a more realistic and sensible depiction of sexual bliss, often found in relatively mainstream works.
Any doubt can usually be dispelled by observing the act being depicted – the involvement of monsters, implements of torture and streams of unidentified liquid usually denotes ahegao material.
Those looking for further material could do far worse than to inspect the relevant tag.