Every year brings a new crop of oddball medical cures. And now, from the collective unconscious that brought you "a bag of water will repel flies" (it won't, it really really won't) comes this new cure for night time leg cramping: a bar of soap. Yes, soap. Doesn't even matter which kind of soap, which is kind of a clue that it is bogus, if you ask me.
It's probably the placebo effect and you should probably get more potassium.
Here is the scenario: you suffer from leg cramps at night. Simply slip a bar of soap between the covers, and your leg cramps will go away. Or, when you get a leg cramp, touch your leg with a bar of soap and the cramps will dissolve. I have heard it said both ways.
Neither method makes a lick of sense, but that doesn't stop people believing it. Just like magnets and copper for pain relief, soap for cramps is one of those folk remedies that people seem to believe without a lick of proof.
People who try it swear by it, of course. There is selection bias at work, here. The sort of person who would try sticking a bar of soap under their blankets is already predisposed to be gullible. And you have a selection bias in the results, as well: cramps always go away on their own. Some sooner than others.
The best way to make a leg cramp go away is to relax and hold still. This can seem almost impossible when you are awakened by a torturously painful charley horse in the middle of the night, but it's true. Lie there and focus on your breathing. My guess is that this is the real magic of the soap: it convinces you that you are doing something, so you lie still and relax, trusting the soap, and lo: the cramp ends.
A better question is, why are you getting leg cramps in the middle of the night in the first place? Any time I have suffered from leg cramps, it always turns out that I am lacking in potassium. The next time you are at the store, rather than buying an extra bar of soap, buy a bunch of bananas. Eat two every day until your cramps subside. Yams, orange juice, potatoes, and yogurt are other good sources of potassium.
Being dehydrated, some medical conditions, and some medications can also cause nighttime leg cramps. If you are experiencing this problem more than (oh, say) once a month, don't buy soap: see a doctor.