If you are talking about those things being sold on TV that claim to detoxify you by magically pulling "bad stuff" out of your feet using magnets, you are wasting your time (and money).
There is quite simply no way for these devices to work as claimed. The only thing you could possibly found stuck to these pads after use is sweat, and whatever you happened to have on your feet prior to putting them on (I assume the instructions tell you to wash your feet before use).
The sweat glands under your skin are only capable of passing a single substance through to the outside of the body, and that's sweat. There are no other openings in the skin (at least nowhere near your feet) that could possibly pass any type of substance through them, let alone after being pulled on by a magnet.
I'm guessing that the only reason these products continue to be allowed for sale in the United States is because the FDA has its hands full with phony medical devices that can actually harm you. As far as I can tell, the worst that could happen to you after wearing these things overnight is a sudden loss of $9.95.