ICR (Infrared Cut-Filter Removal) works in two ways. On box cameras the the filter sits in front of the CCD module. The filter is rectangular in shape and is split into two parts. One side is IR sensitive the other blocks out IR to achieve the correct colors. It is powered by magnets to switch it on or off. When there is an electric field it will slide the correct side over. This is controlled by the separate ICR chip. The chip is activated by an external CDS sensor. So when it gets dark then the ICR is activated. Its kinda like the ones that turn on the LED's. I work for a camera manufacturer and i repair these all day. These are the first things that go bad on day/night cameras. I'm guessing because of the mechanical nature of the ICR.