It is a feature that was much requested after this behaviour was changed in 1.4. A tap send the device to sleep--a 3 second press on the power button turns it off. Once asleep it stays asleep--no more turning itself off after 15 minutes.
Kobo reported that after extended testing, the sleep option was not causing as much of a battery drain as they had thought it was so they essentially returned to the pre 1.4 behaviour (with the option this time to show covers (or not) for both sleep and off).
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