Amazon Fire TV devices never really turn off. Even when they go to sleep after not being used for a while, or when they are manually put to sleep, they actually just black out the display but remain powered on. This is so that they can instantly be ready to use and so they can perform software updates while idle. For Fire TV remotes with a power button, pressing it just turns the TV off and does nothing to the Fire TV itself. Anyone that wants to actually power down their Fire TV must pull the power cable. However, the one exception to all of this is the Fire TV Cube, which is the only Fire TV model that can truly be turned off without needing to unplug anything.
To turn off a Fire TV Cube, you simply hold the Action button on top of the device for about 10-15 seconds. The Action button is the right-most button with a dot in the middle. This works on both the 1st-generation and 2nd-generation models. Doing so will indeed shut down the device so that it is using no power. Since all other Fire TV and Fire TV Stick models don’t have any buttons on the device itself, they cannot do this. To turn the Fire TV Cube back on, simply press the Action button once and it will boot up, as if it were unplugged and you plugged it back into power. While truly off, the Fire TV Cube will not respond to any remote button presses, so you have to use the Action button to turn it back on.
I don’t expect many Fire TV Cube owners will do this regularly, but it’s good to know it’s possible if you won’t be using the device for a while and want to save some power. The 2nd-gen Fire TV Cube is the fastest model to power on, at about 30 seconds, so it’s not too inconvenient to turn it off with this method if you don’t mind walking up to the device. The 1st-gen Fire TV Cube takes just over a minute to power on, so that’s a bit more time to wait.
The biggest disadvantage to turning off a Fire TV Cube regularly is it may feel a bit sluggish for a while after being turned back on. This is because the device couldn’t perform updates and maintenance tasks while idle so it may be doing more in the background while you’re actively using it than if it was left on but asleep.
Thanks to a great comment from Heath, it turns out the Fire TV Cube is technically not fully powered off when in this state. Heath astutely points out that the red microphone disabled light remains lit after shutting down the Fire TV Cube using this method. So, holding the action button for 10-15 seconds is more akin to putting the device in a deep suspend state where all processes are shut down and very little power is used, but is not a fully off state.