"Rub Rub", as it is referred to by Yukkuris, is one of the main physical interactions between practically all Yukkuri. As the name suggests, it's when two (sometimes more, if it is between a parent and their koyukkuri) yukkuri rub together. It appears to serve two functions at times.
Firstly, the Rub Rub is a sign of affection, a bonding activity. It's important for a baby and koyukkuri to rub with their parents, in order to develop and strengthen the bond between them. Failing to do so will often result in rejection of the parent, the child declaring it as a "useless parent" that is unable to take care of its young (in rare cases, parents that are unable to take it easy). Grown yukkuri that are partners will also rub together. Also, koyukkuri playing together often will stop and do rub rub as a sign of affection and happiness, and pet koyukkuris, caring for their Mister Owner as a parent figure, will actively seek to rub rub (due to size different, rubbing with the Anon's hands or being petted and caressed) with him.
If there's no other activity that the yukkuri can engage in (e.g. during winter months when it's unsafe to leave nests), the Rub Rub is also used as entertainment and a time-passer activity. It could be compared to that of a human hug. In the cold months, rub-rub is also used to help yukkuri keep warm, similar to a human rubbing their hands together.
The Rub Rub is the original, and initial way of Yukkuri reproduction. A much more vigorous version of the "affection" rub, two Yukkuri will rub together and exchange a "Paste bean seed". Reproduction in this method appears to always result in a stalk-type birth. The Peni Peni method is now much more common, but the Rub Rub method is still used.
Since Yukkuri mostly lack restraint and are instinctual creatures, once they start an Affection rub-rub, or they start rub-rub to keep themselves warm, they're often unable to resist climaxing, thus going in no time from the Affection rub-rub to the Reproduction rub-rub. Since it always results in pregnancies, an Anon unwilling to care for a large family should avoid giving yukkuris the occasion and the need to rub-rub for prolonged periods of time, avoiding to keep them in colder climates.
As such, while it's still considered part of the average yukkuri behavior when two Yukkuri spend time doing rub rub, an increase in the frequency and intensity of rub-rub is a clear signal that the couple is developing a desire for parenthood.