Although it's obviously limited on everything it can do, apparently the Fitbit app has been upgraded so that it no longer requires the actual wearable for many of its functions. You can actually get several of its capabilities directly with your Nexus 6P or Nexus 5X.
The app uses the dedicated Sensor Hub chipset found in these devices to help detect various movements which suggest activity. Here's a quote with the details,
"It's called the Android Sensor Hub and acts to monitor the device's movement, not unlike the co-processor in the new iPhone 6s. The dedicated processor connects directly to the camera's sensors to reduce the phone's overall power consumption. This chip allows the phone to run advanced activity-recognition algorithms that, say, notice when you start a run or hop on your bike -- even if you haven't turned on the screen." ~ Engadget
Here's our dedicated sections for discussions on both of these devices:
