Scientifically formulated pellets inside the Wash-It Ball 'activate' water molecules producing electrolytic oxygen and hydrogen ions, which unleash their natural power to lift dirt from clothing fibres.
My mum first put me on to them. She used them in my parents' holiday home in Ireland because it has a soakaway, so she couldn't use her usual detergent. In my experience they do a good job of cleaning most clothes at 40degrees or less. They don't quite stand up to tough dirt (such as baby clothes with lots of food stains, or children's clothes with mud and grass stains) as well as the super-dooper powders with enzymes and what have you. So I tend to keep some stain zapping products around for those cases.
I like the fact that they leave your clothes smelling just clean, rather than full of perfume. But the best thing about them is that they're good for about 100 washes so at £7.99 each they work out very much cheaper than Persil, as well as being better for the environment (well, I'm not so sure about the plastic ball). And that leaves us with more money to pay off our mortgage and one day buy some land.