What is "instant on?"
It is a setting on the Xbox One that let's it go into sleep mode and resume very quickly. 10-15 seconds instead of almost a minute to boot up. It also allows it to download software updates as soon as they are available, even if you are not using it. Oh, and to turn on when you say: "Xbox On."
BTW, furry, I just checked the One again tonight and my NAT is still open using instant on and default settings on the 5500.