Naturally I assume it requires some extra bit of battery...but probably such a very tiny amount since these new chips are designed to sip power while being aware for certain actions.
Only way I see it draining the battery is if we accidentally wake the phone. The concept is awesome and very convenient but I noticed I sometimes inadvertently wake it.
From my understanding, the m8 has separate chipsets to monitor small computing tasks such as fitbit, motion gestures, etc. Therefore, the battery drain is negligible.
I figured since it was based on the gyroscope it would always be on while we walk around and move the phone. Maybe not. Also if it does have a separate chip set that would help.