If it were to be converted to a 1-bit image (Black/white) you could still make out what it is. That's basically the silhouette test.

This one doesn't quite work because there's no separation between the three colors that make up the circle. Not that it needs to, though. This one is better than the last for this test. In this case the silhouette would be the larger outer circle and the smaller inner circle.

It's really only used in print design, or at least that's how it started because logos needed to be versatile enough to work in many different color spaces, including being sent via fax. It is useful to keep in mind so that we create clean and uncluttered logos, though.