Geisha Balls: The Ancient Traditional Japanese Art of Vaginal Workout
Apparently coming from an old Japanese tradition (hence the name), geisha balls are two small balls made of plastic or thin metal (ivory, gold or silver for the more traditionnal). One of the two usually encloses a third smaller ball or is partially filled with a heavy liquid, such as quicksilver, in order to increase its momentum whenever the balls are moved around.
These balls, as a pair (optionally tied to a small thread to ensure easy removal), are then inserted into the vagina and kept there while attending business as usual.
The first consequence is quite obviously a direct arousal of erogenous zones thanks to the friction and the vibrations created by the smaller ball bumping against the wall of the hollow one. This is apparently a rather pleasurable experience although not to such an extent that it would prevent normal everyday-life activities.
But the real purpose of Geisha balls is in fact to develop the outer vaginal muscles (in a fashion similar to Kegel exercises) and increase the ability to control vaginal movements. This "workout" allows both partners to enjoy considerably increased sexual experience: longer for the man, leading to stronger orgasms for the woman.
Although I was not able to find much documentation on the history of this practice, it seems the technique was discovered by a woman named Rino-Tama and especially prized by Geishas (maybe still is, but honestly, I wouldn't know)... At any rate, it is still popular in Japan, and probably other parts of the world too (a friend working for a famous San Francisco-based female-oriented sex-toy store confirmed it can also be found under slightly different forms in the US and elsewhere).
With a bit of training, women are not only able to merely hold the balls in their vagina, but also literally do "tricks" with them, such as releasing and sucking them back in, although this kind of skill is probably not necessary for the balls to fulfill their purpose.
I can only encourage curious people out there to try this rather unusual artifact with their loved one (I would not try with pool balls or any items not specifically designed for this purpose, though).
Geisha balls are sometimes also called Ben-Wa Balls and come in all kinds of shape and size; modern-day variants in Japan include smoother materials and built-in vibrators... I would not be surprised by the availability of a "Hello Kitty" version...