We have been covering various 3D Touch features extensively here on iDownloadBlog and will continue to do so in the foreseeable future.
In addition to using your iPhone 6s keyboard in trackpad mode, adjusting the sensitivity of 3D Touch and turning the feature on and off, iOS 9 also provides a way for iPhone 6s owners to use its built-in zooming capability in conjunction with 3D Touch, based on the amount of force exerted on an on-screen controller.
They're calling it Peek Zoom and once activated lets you invoke iOS's standard Zoom feature through 3D Touch rather than requiring you to double-tap with three fingers.