Placing finger twice on Finger Scanner on iPhone 6 plus changes view
This is called "Reachability" and is Apple's way accommodating allow for one handed use where the thumb can reach the top of the screen. 

It exists on the iPhone 6 line where few people have hands large enough to reach the top of the screen when holding the device from the bottom. Smaller devices such as iPhone 5s and older do not have this toggle. iPads also lack this control.

It is described here: https://www.apple.com/iphone-6/design/