I think I found it.
PRS-600 User manual:
http://www.sonystyle.com/wcsstore/So...set-166216.pdf
Page 11
"...
To turn off the Reader
Slide the POWER switch again.
The Reader turns off. After turned off, the Reader enters
sleep mode.
When turning off the Reader, if you slide the POWER switch
and hold it for more than 3 seconds, the Reader shuts down.
..."
Page 12
"
To shut down the Reader manually
You can shut down the Reader manually in one of the
following ways.
Slide the Power switch and hold it for more than
3 seconds.
Use “Device Shutdown”
..."