Idolmaster: https://booru.allthefallen.moe/posts/318383
KanColle: https://booru.allthefallen.moe/posts/260972
Unifying theme is that the guy is kneeling, the loli is face-up (supine, not prone) pelvis is airborne, thighs are horizantal, but she is bending backward. This logically causes the whole stomach bulge phenom (which I still think ought to be retitled belly bulge... maybe when we get tag tools?)
Variations exist like whether or not the loli is making contact w/ surface behind her, and if so, via which body part. In the Idolmaster example Yukimi is supporting some of her weight w/ her palms, while in the KanColle example Asashio is supporting some of her weight w/ her elbows. These variations are minor enough I would still consider them the same posture.
I thought about whether piledriver would cover this, but what I think is different there is with a piledriver the knees are held up higher than the pelvis, producing more of a "completely upside down" position, whereas what I'm talking about isn't quite as extreme. We're talking a supine pelvis which would only bend the penis about 90 degrees, and not the 180 degree cartoonish bend necessary in a piledriver (that tends to be pretty painful if you're fully erect)
"Penile drop" as discussed in https://www.menshealthhandbook.org/erection-pointing-up-down seems to be necessary for the cartoonish "piledriver" pose whereas the 2 images I posted at the top seem to recognize that ligaments pull it upward and that there would be a lot of resistance (thus the belly bulge) pressing against the anterior wall.
Sometimes there are even "beyond 180 degrees" piledrivers such as https://booru.allthefallen.moe/posts/101905 though it can be hard to perceive since you need to envision the pelvic tilt going on, the bone which is bending the boner