Well, looks like these feature is supposed to work along with the Playback Limit setting.
I was assuming the most obvious behaviour, but looks like it doesn't work in the most expectable way.
So, when you have Playback Limit set, and that time is reached, the box will enter the selected Power Mode.
The expected behaviour would be that after some time of inactivity (not from the Playback Limit setting), the box would enter the selected mode. Or at least should be explained somewhere that these 2 settings work together!