You need to go to Settings > Google > Search & Now > Voice > "OK Google" Detection and enable the "Always On" option and train your voice to it.
It works for me on Verizon... Not sure if that matters.
Anyone have any information about what this does to your battery?
No difference.I was wondering the same thing. My phone has been losing 1% per hour overnight. I have disabled the always on option to see if it changes anything. We'll see tomorrow morning.
I thought it only worked with the screen off when it was charging.
I have had the always on option activated since 3 days after getting the phone. Battery loss is really negligible..