This is another feature buried in the Accessibility section. Navigate to Settings > General > Accessibility and then scroll down to the Interaction subhead and tap "Switch Control." Tap "Switches" and "Add New Switch." Select "Camera" and choose "Left Head Movement" or "Right Head Movement."

Then pick an action under the "System" menu, which essentially tells you what task your head movement will trigger. I chose Siri, so each time I tilt my head to the left it automatically launches Siri.