Once enabled, Reachability can be invoked by swiping down on the bar that serves as a visual indicator for returning to the Home screen.
Skip to the 5:05 mark in Engadget's first impressions video to see Reachability in action. The tidbit was highlighted on Reddit by user Whert Ser Nert.
Reachability is designed to bring down the entire top portion of the iPhone's display so it is within thumb's reach for one-handed access to apps and content. The feature is invoked by double tapping the Home button on other iPhone models.