Apple already implemented that on their current phones with IOS8 beta. The only time it works, it is when the phone is plugged in and charging...that's why that it doesn't affect the battery or need any new specs...etc.
So for example, for people that has a car charger setup with a cradle, or using the Nokia charger, then when we would be in the car and the phone is charging, only at that point Cortana would listen. So while driving, no need to press any button at all...just say Hey Cortana....etc.