I was recently browsing the GIAC site (looking for good writers in security – if you know any, have them drop me a line) and came across a paper on Port Knocking by Ben Maddock. From the Port Knocking Website:

"Port knocking is a method of establishing a connection to a networked computer that has no open ports "

This is a pretty clever way for a system to "maintain a low profile" by inverting a challenge and response process where the challenge is someone knocking on the "door" and the response is to "let them in" by opening a port.