The experimental feature, which was more than likely highly requested by users, allows your Fit to continuously track your activity, even when it is unpaired from a smartphone. For example, if you go on a run through the woods and leave your phone in the car, your watch should still track the data properly, then sync that info once your devices are reconnected.
Below is the changelog from Google, so be sure to go grab that update.
- Support for manual entry of over 100 new activities!
- Android Wear – New, experimental step detection for walking when the watch is disconnected from the phone
- Bug fixes and performance improvements