Amazon says that Alexa can be invoked when a Fire 7 or Fire HD 8 tablet is plugged into a power source with its screen on or off. If the screen is off, the tablets will only respond if they are plugged into a power source. You will not be able to access hands-free Alexa if the device is asleep and plugged into a low-power source (Amazon uses the example of a computer's USB port).
Interacting with a compatible Fire tablet works in pretty much the same way as you would expect with a dedicated device like an Echo or Echo Dot. So, you can use the tablet to control your smart home, fetch weather reports, or even playback music. And given that the Fire 7 and Fire HD 8 both have displays, they are capable of accessing video feeds and other video content that you might be used to viewing with say, an Echo Show.
Amazon notes that if you have more than one Alexa-enabled device in your household, Alexa will still intelligently respond on the device that is closest to you using Echo Space Perception (ESP) technology. This will prevent the undesirable effect of multiple devices responding to your every command.
Enabling Alexa hands-free on supported Fire tablets is as simple as swiping down from the top of your device's home screen, and pressing the "Alexa Hands-Free" icon from within the quick actions menu.
The Amazon Fire 7 (8GB) is currently available for $49.99, which makes it the same price as an Echo Dot for roughly the same [Alexa] functionality. The Fire 7, on the other hand, has the added benefit of being more useful as a consumption device when not being used as a digital butler.
Last month Amazon extended Alexa voice calling to its Fire tablet and the Apple iPad.