Aleksandar Vacić of Radiant Tap has written about the process behind the iOS 9/watchOS 2 update for his app, Run 5k:

Run 5k for watchOS 2 will tap your arm when you need to change pace during the training run thus not interrupting your music, ever. And at the end, you will see your results right on the watch app.

When you get home, you take the Watch off, place it anywhere close to your iPhone and while you shower all the data will be synced over. You can check your results in either Run 5k or in Apple’s own Activity app. Or in any other 3rd party app that’s leveraging Apple Health platform.

It’s not just that they’ll be faster. watchOS 2 apps will have a deeper access to Apple Watch hardware that will start making them more independent from the iPhone and personal – truly, as this is the kind of data that not even an iPhone can simulate on its own. I have a feeling this will be a massive change for fitness and health apps on the Watch this Fall.