Pauline Reage probably did as much as anyone to popularize butt plugs, even though she didn’t really understand how they worked. It’s not the physical stretching that does the job, it’s getting accustomed to the surrender:
For the next eight days, between dusk when her stint in the library came to an end and that hour of the night - which was generally eight or ten o’clock - when she was returned to her cell, in chains and naked beneath her red cape, O wore an ebonite shaft simulating an erect male member which was inserted behind and held in place by three small chains connected to a leather belt around her hips, in such a way that the internal movements of her muscles could not expel it. One little chain followed the furrow of her buttocks, the two others the fold on either side of the belly’s triangle, in order not to prevent anyone from penetrating that side if need be.
….They all had one feature in common, namely that they flared at the base, to make it impossible for them to slide up inside the body, an accident which might have produced the opposite effect from that desired, that is it might have allowed the ring of flesh to tighten up again, whereas the purpose of the shaft was to distend it. Thus quartered, and quartered each day a little more, for James, who made her kneel down, or rather lie prone, to watch while Jeanne or Monique, or whichever girl happened to be there, fastened the shaft that he had chosen, each day chose a thicker one. At the evening meal, which the girls took together in the same refectory, after their bath, naked and powdered O still wore it, and everyone could see that she was wearing it, because of the little chains and the belt. It was only removed, by the valet, when he came to chain her to the wall for the night if no one had asked for her, or, if someone had, when he locked her hands behind her if he had to take her to the library. Rare were the nights when someone did not appear to make use of this passage thus rapidly rendered as easy as, though still narrower than, the other.