There are a number of reasons why critics are saying Fifty Shades Darker surpasses its predecessor, but the most compelling points to a pair of silver spheres known as Ben Wa balls. The BWBs, as we’ll call them, spent most of their time nestled in Anastasia Steele’s “most private area,” as The Guardian coyly pointed out in a list of the film’s most ridiculous moments. There is, however, nothing ridiculous about BWBs, which have a long history of inducing sexual pleasure and giving vaginal muscles a serious workout — which are not mutually exclusive events.

When they’re used as sex toys, the balls, which range from one inch (normal size) to two inches in diameter (“Extra Large Extreme”), are pushed into the female vagina all the way back to the cervix, the opening at the end of the canal that leads to the uterus. They usually come in a pair — one hollow and one slightly weighted — and the lighter of the two is inserted first, followed by the heavier one, which anchors the pair in place. There, they sit comfortably and vibrate as the body moves. These vibrations are thought to induce gentle waves of pleasure, as Ms. Steele illustrated by shuddering with every step she took at the masquerade charity ball. The waves are not so intense, however, that they cripple the bearer with orgasms — which explains their original function: to tease.

Ben wa balls, also known as Kegel balls, are used to induce low-level sexual stimulation and to exercise the pelvic floor muscles.
“I’m not putting those up my butt.”

Teasing is a key part of Christian Grey’s sexual strategy, and it’s a scientifically sound one as well: A healthy amount of foreplay — all of the arousing activities leading up to sex — was shown to be a key factor in successful long-term relationships by a study in the Journal of Sex Research in 2016. Though their origin is hazy, it’s thought that BWBs were originally developed in order to practice sexual discipline by followers of Taoism, whose beliefs about sex largely center around the controlled release of sexual energy.

Relatedly, the balls are very useful for strengthening the muscles on the pelvic floor (colloquially known as Kegels), which in turn are involved in experiencing pleasure during sex. When they’re referred to as Kegel balls, they’re meant to be used as a tool for vaginal weightlifting: Because the weighted ball falls toward the vaginal opening while a female is standing, she must continually squeeze her Kegel muscles to keep them in place. Giving these muscles a workout increases blood flow to the vagina during sex, improves a woman’s ability to reach orgasm, and increases lubrication and tightness of the vagina, according to a Memorial Sloan-Kettering Cancer Center report. A bonus is that the same muscles are used in holding in urine, so the balls also keep incontinence at bay.

Grey is, by nature, not exactly a thoughtful and generous lover, but by slipping Steele her first pair of silver BWBs, he provided plenty of potential for sexual foreplay and a vaginal workout to boot. Good thing, however, Steele speaks up and says, “I’m not putting those up my butt,” because putting them in the anus is, as the Ultimate Guide to Ben Wa Balls puts it, “a very bad idea.”