The sensor isn't much bigger than the tip of a ball pen and has three infrared light sources and a sensor inside. The three lights are arranged in a triangular "v" pattern and pulse briefly on and off in sync with one another so more than one light is never on at the same time.

It's this triangular arrangement and timing that makes gesture detection possible.

Consider a swipe from right to left, as a tablet user might make when turning the page of a book or jumping to the next photo in an album.

When the user's hand is brought over the sensor, infrared light from the right-most of the light sources is reflected by the hand first, followed by light from the source at the bottom of the "v", then light from the left-hand most source.

By analyzing the order the sensor can determine it was a swipe from right to left.