I remember when I was very small and I got my first TV game which included the light gun. And I thought: How can the TV detect when I hit the white square on the screen and pass this information back to the console? What kind of magic is this? Of course, the "magic" was that the whole "shooting" actually happened backwards. The TV "shot" its light and my gun just detected it.
It's somehow similar with Nintendo Wii and its innovative controllers, as the guy in the following video explains (rather erratically). The "sensor bar" is actually not "sensing" anything at all, it just contains two sets of infra LEDs and the sensors (infra cameras) are inside the Wiimotes! That's why they are so expensive. This means you can theoretically place flower pot or your cat in front of the sensor bar and - as long as the ends of the bar aren't obscured - the Wiimotes will work perfectly!