It's not a power switch - it's a "sleep" switch (you can't turn a Kindle off). When you put the Kindle to sleep, all the buttons other than the "power" switch (yes, I know Amazon call it that
) are disabled, thus preventing accidental page turns, etc, if you put the Kindle in your bag. There's no power saving in putting the Kindle to sleep - WiFi is still on (if you have it enabled).