Time for some Science with iFixit!

Just how does the Fire Phone know where your face is in relation to the phone and create that fancy 3D effect?

There are four IR projectors, one near each corner of the phone. They're blasting you with invisible (to the human eye) infrared rays any time you're staring at the phone while the display is on.

There are also four IR cameras capturing this information—one at every corner of the phone.

The Fire Phone triangulates rectangulates your position, figures out that there's a human face looking at the display, and starts working its wonderful juju magic—all in real time.