laundry balls/disks

Since I'm currently trying hard not to develop a sensitivity to borax, my current detergent, today I investigated laundry balls and disks. I'd never heard of them, but rumor has it that they somehow isolate dirt so it can be rinsed away, just like detergent does, but without a detergent. Yeah, I know, those hydrophobic/dirt-philic molecules we learned about in class don't just pop in out of thin, uh, plastic.

I didn't research any farther than The Straight Dope, which reports that laundry disks work probably by the same principle as whacking your clothes on rocks.

Rocks worked fine for thousands of years, so I have no problem with that. Rocks are cheap.