When you are on a bike at a traffic signal, look for the T. When you put your front wheel over it, it tells the signal you are there, the same way that a car driving over the signal detection loops tells the signal they are there. It doesn’t mean it will immediately trigger the signal to change; but makes sure the signal knows you are there waiting your turn too.
Bicyclists have asked “does it matter if your wheel is right on it? Does it matter what your bike is made of?” The bottom line is no. The metal parts on your bike are what is detected. In the future we are moving to a national standard with a bike symbol with a modified “T’ instead of the stand alone T’s. 
(that should make it more obvious for cyclists). A colleague just sent us this video from the City of Redmond. It’s pretty good! It shows the future symbol, as well as explains how to look for the “X” and how to hit the “sweet spot”.
FYI, all jurisdictions are required by a new state law that after July 2009 all new vehicle activiated control signals must detect bicycles and motorcycles. Implementation of the law is subject to funding. We’ve gotten started….so look for the T or bike symbol and “T it up”.