What unusual input feature did the original PlayStation Vita have on its back?

The original PlayStation Vita had a rear touchpad on the back of the handheld.

The capacitive rear touchpad was one of the Vita’s distinctive control features. Games could detect touches and gestures made behind the screen, allowing developers to create actions that did not require covering the front display with a finger.

The Vita also had a front touchscreen, dual analog sticks, motion sensors, cameras, and traditional PlayStation buttons. These inputs gave developers more control possibilities than the earlier PlayStation Portable, which had a single analog nub and no touchscreen.

The rear touchpad should not be confused with a rear touchscreen: it did not display images. It was an input surface, and its use varied substantially between games. Some titles made it central to gameplay, while others barely used it.