Why does my iPhone screen drop down when I tap the home button?

Episode 1317 (1:54:28)

Doug from Long Island, NY
Apple iPhone 6s

Doug has iOS 9.5.3 installed, and when he taps on the home button, the whole screen drops down. Leo says it's a feature that allows you to use your phone one handed because the iPhone 6S is larger than the previous iPhones. It's supposed to do that whenever you double tap the home button, without actually pressing it down.