"Services focused on tracking health will be able to use the Watch interface to display relevant, up-to-the-minute statistics in a way that’s more convenient than on a smartphone, or on a monitoring device’s screen. It will do this using the processing power of your iPhone, rather than a mobile chip onboard the watch itself, and updates will be sent to the watch wirelessly." - http://www.wired.com/2015/02/apple-watch-apps/