This little clip is designed to hold a FitBit Flex to a change pocket, shirt pocket, or other fold of your clothes, where you can easily see your progress without fishing around for it. The tracker snaps in securely with a satisfying click, and can be pushed out through the back
This video shows how the Fitbit snaps in and out. It shows a different model, but the pocket clip works the same way.
There's now a similar clip for the Fitbit Alta!