Kindle's CANNOT be turned off. There is no ON/OFF switch.
The wireless system CAN be turned off.
Sliding the switch and holding for 10 sec puts a screensaver on the screen. This has NO impact on the kindle being ON/OFF.
On 6 flights I've asked pilots in front of the FA's if it's OK to read a Kindle since it does not have an ON/OFF switch and all I can do is close the cover. 4 pilots have said reading them during takeoff/landing is find but just make sure the wireless is off.