"Slipping a bar of soap into the bed as a leg cramp prevention has been advanced by a number of authorities, both medical and otherwise. ... As to how this works — or even if it does — we're still in the dark. Perhaps soap releases something into the air that is beneficial to those predisposed to this condition, with the bedsheets working to contain the helpful emissions to the area where they are needed. Or perhaps this is a case of believing making it so — the soap itself has no effect, but the sufferer's faith in the procedure serves to effect the miracle." (Snopes:
Soap Dope)