The common explanation is that having something under the covers that keeps gettin' in the way encourages you to move a little and kick it around occasionally, and prevents you from blocking the circulation for too long at a time.

It doesn't work for us, but that's probably because any time you try to move your foot there's at least one cat asleep on top of it that has it pinned down. You have to wake up enough to kick pretty hard, which sort of defeats the purpose of the soap.

As a side note, a recent TV show called "The Doctors" that generally gives pretty good advice on stuff recently claimed that:

"recent laboratory testing has shown that the claim in folk wisdom that a sip of pickle juice will stop leg cramps does work, but nobody has any idea why."

John