Lauren Goode:

A new feature called “SmartTrack” will now automatically detect when you’ve started a workout session or new activity, rather than requiring you to press a button or swipe through an activity menu to start a workout. According to Fitbit, the software is able to detect running, outdoor cycling, walking, elliptical workouts, basketball, tennis, soccer, and even high-impact cardio workouts like kickboxing and dance classes.

Still waiting for HealthKit support. Get it done please, Fitbit.