Apple's implementation of a "solid state" home button expands the Taptic Engine's feedback on the iPhone, and a forthcoming API for developers' use will expand implementation to non-Apple apps.
During the iPhone 7 announcement, Apple executive Phil Schiller showed off the new force sensitive home button, which is now not a mechanical button. Instead, the button is pressure-sensitive, and utilizes Apple's Taptic Engine for feedback for the user.
Apple's Taptic Engine debuted with the Force Touch feature in Sept. 2014's unveil of the Apple Watch. Since initial reveal, it has been implemented in the 2015 MacBook rebirth, the MacBook Pro, and the Magic Trackpad 2.
Making the API available to developers to use will allow for better force feedback implementation OS-wide, and give developers a way to utilize the feature in other ways, such as haptic responses to actions in a game.
Taptic APIs have been available since iOS9's release, but have been under Apple's lock and key.
The previous "UITapticEngine"
Previously, the Taptic APIs were private in iOS 9, and limited to Apple-only use. Some enterprising developers examined it in some depth, but applications submitted with private APIs are rejected during the App Store review process.
Schiller's new API discussed at the iPhone 7 reveal is not yet available to developers in the current iOS 10 and Xcode 8 "golden master" releases. However, Apple's approach of last-minute API releases, or releases in a point update of iOS and Xcode, is not unusual.
"[A new Taptic API in the beta releases] would have revealed a new feature," developer Rosyna Keller told AppleInsider. "Apple hates that, they tend to scrub it from public seeds until the next major public seed."
At present, even the previous, private Taptic Engine API's have been excised from the pre-releases. More information on the hooks for the feature for coders will have to wait on Apple's official documentation.