I don't entirely believe those actually act like period cramps. When I use tens machine type things above a certain point all I really FEEL is a zapping sensation because of imperfect connections to my skin. And those HURT. But the actual muscle twitches are just muscle twitches.
My impression is they don't REALLY act like period cramps, it's just as close as we have tech to get because the TENS machines cranked up can cause pervasive discomfort or pain. So it's not so much that it's simulating the actual feeling as the level of discomfort/pain that people experience.
They're just tens machines? Huh. Yeah, I've used those for stiff muscles and such, wouldn't have figured they could simulate cramps of any kind.
I don’t think it matters so long as it’s significantly uncomfortable and outside of the wearer’s control. We did an exercise in a long ago birthing class that involved plunging hands into ice water. Not at all like labor contractions, but still very instructive for the partners without uteruses.
I must admit I assumed they were something else... though obviously related....