The Xbox One's voice commands, while generally excellent, have a problem. OK, two problems. The smaller one is they don't always work. The bigger one is that you need to memorise a chart like this just to be able to use them properly.
I will never understand why you say "Xbox on", but not "Xbox off". Mr Miyagi would be very disappointed.
And maybe I've been spoiled by the excellence that is Google Now, but I'd hope that in future updates, the camera can be a little more forgiving, with less reliance on having to say exact terms.