It works by finding an exact copy of an image you specify, either via upload or via a link. It does not find near images, the result is binary - either the image matches or it doesn't. Some small differences are allow for example, searching for Mona Lisa in curlers will bring up the original version of the Mona Lisa. One interesting part of the service is that if you provide a web page, it will load the images, and allow you to click on them to conduct a search. The service is useful for tasks which require known item search - e.g. I've got this image, has someone used this before on the web, I need an original image.