When an image alludes, spoofs, or otherwise parodies the real-life familial relationships of the racehorses that the Umamusume girls are based on.
The Pixiv tag's name refers to the complicated bloodlines of racehorses as a result of their breeding procedure, in which a single sire has several partners, with incestuous pairings being common to further strengthen the strong bloodlines.
Note that the only familial relationship that is acknowledged canonically in Umamusume is the one between siblings (born from the same dam). A good example for this is the V Sisters. In their case, it would only be an example if any of Halwa Sweet, Deep Impact or Heart's Cry are also present.
So far, there are many father-child pairs (e.g. Rudolf and Teio) but only three mother-child pairs in Umamusume: Fusaichi Pandora and Almond Eye as well as the Air Groove, Admire Groove and Duramente mother-mother-child chain.
Pixiv equivalent tag: ホースの暗黒面 or 史実親子 (parent and child in real life)
(Yes, it's also a Star Wars reference.)
