The Wired article
here.

For the first time, motion sensing occurs in a separate processor,
which makes constant activity tracking using the gyroscrope and accelerometer
sensors more power-efficient without turning on the rest of the A7 chip. This
means we’ll start to see more Quantified Self (QS) tracking apps detecting steps
and stair-climbing, bringing Fitbit and Jawbone capabilities to our phones. And
the M7 does all this without a noticeable drain on the battery.

But that’s the sticking point: noticeable. The introduction of the M7 means
Apple could collect this activity and movement data in the background without
affecting our iPhone experience. Which introduces the distinct possibility that
this data is being collected — whether or not we are opting to turn on an app
that makes that data legible to us.

Now, with the M7, activity tracking comes as an automatic feature on the device
that most of us carry with us all day, every day. It’s one thing to actively
decide to wear a fitness-tracking device. It’s another to get one — by default —
as part of the exchange when you buy into the iPhone. It gives Apple the ability
to capture that activity data passively, whether we are interested in it or not.
(They’re not the only ones, of course. The M7 is similar to the coprocessors
coming out in Google/Motorola’s Android-based Moto X as well.)

Thanks to Russ Brown.