Hilarious now, given that although the label is usually identified with Sanae types, it applies more to Reimu types with how yukkuri are portrayed in fandom.
Onbashira-Re: Sanae--!! What's going on?
Sa: Even if you ask "What's going on," I don't know. (troubled)
Re: And here Reimu trusted Sanae... it's horrible!! (angry)
Sa: Horrible... I don't understand what happened, either!! (crying)
Allow me to explain. The "Yukkuri Sanae" is a rarity among rarities, born as the offspring of a "Yukkuri Suwako" and a "Yukkuri Kanako," but the Sanae's birth rate is one in ten thousand, and since her body is not strong, it is extremely rare for a Sanae to grow to adulthood. (However, if one reaches adulthood, through the power of miracles, she is extremely long-lived.) Lastly, when a Sanae mates with a different yukkuri, only "Suwakos" and "Kanakos" are born--no matter what type of yukkuri the father yukkuri is, that type will not be born. (The Sanae herself is unable to understand why this happens.)
Incidentally, for some reason, Alice-types are the only ones who understand this phenomenon, and a Sanae is the only type of yukkuri that the notoriously sex-crazed Alice will not mate with.