As a Moto X user, I heard that phrase in the keynote too and wondered myself. For me, this is the killer feature (combined with Moto Assist) that keeps me locked into the Moto X -- I can't live without it. I don't *do* anything to my phone, it detects when I'm driving (and when I'm not) and prompts with voice to answer calls, reads texts, etc. No touching the phone. Works when screen is off, when phone is in my pocket (forcing me to yell) or more frequently when phone is running Waze in a phone mount.
@FunnyHouse, I've never had a mis-trigger (it seems specific to my voice, my kids/wife usually can't trigger it whereas it works for me on the first time every time) and battery consumption is not an issue (but I think that was due to Moto X hardware specifically designed for the feature).