Answer: The ben-wa balls sold by many sex toy stores are nothing like what women in many Asian countries used in the past. Actually, we are not exactly sure what they used. Most of what we know is based on rumor, not proven fact. The women may have strengthened their pelvic muscles by inserting objects for their vagina to grasp, as a training weight, but they may not have masturbated with them.
There are "ben-wa balls" that are nothing more than gold plated ball bearings, which serve no sexual purpose, they are novelties. The vagina is usually not sensitive enough for objects inserted to be sexual stimulating, especially if they are stationary and not producing friction. These objects may provide just as much stimulation as a tampon, none. The vagina is a potential space, so objects do not roll around inside it, as the vaginal muscles will grasp anything that is inserted. The weight of the balls may result in their falling out of the vagina, especially if you can't feel them.
They also sell a pair of plastic balls that are 1 1/2 inches in diameter that have another smaller solid ball inside, that are called Duotone Balls. Women are not likely to be aware of the balls rocking back in forth inside, except during vigorous movement, and even then just barely. There are women who enjoy inserting them because they provide something for their vagina to grasp, and fill their vagina during sexual arousal and orgasm. Others use them in combination with a vibrator, as the inner ball vibrates against the outer ball.
They make vaginal barbells and vaginal weights for women to use to strengthen their vaginal and pelvic muscles. Look on the Links page, under Incontinence, and at Bettydodson.com for additional info.