This Lora depicts the act of inserting one nipple into the other breast, with the nipples facing each other. Frankly, it's difficult to differentiate this from “nipple-to-nipple” or “symmetrical docking”.

As a phenomenon, it remains a state where the breasts are pressed together. However, the occurrence of “nipple penetration” is the defining characteristic of the “reverse nipple penetration” depicted in this Lora.

The trigger word is ‘revnippen’. Including ‘2girls, yuri, breasts, nipple-to-nipple, nsfw, mammary gland, cross-section, x-ray, lactation, nipple penetration’ in the prompt tends to yield relatively good results.

Gentlemen and ladies with twisted fetishes, please make use of it.

(Addendum)

I recommend including ‘penis’ in your negative prompt. The ‘cross-section, x-ray’ prompt appears to be deeply associated with “penis” and ‘cervix’. While including it in the negative prompt may still result in their depiction, it increases the likelihood of obtaining the intended description.