Uterine stimulation. It's byproduct of ero-artists imagination by the same logic that pounding the cervix feels good for the woman.
Haha lol
This is super cool!This is bad!
This is bad!!Wh- what is this..?♥Th- this... is ridiculous...Ah... stop... don't!
I'm cumming♥
I'm cumming... I'm cumming♥