On the vocals track, when you build up enough credits for hitting perfect sequences, gold-shaded areas will appear on the track. When you hit one of these yell (you can shout anything) to activate Overdrive.
I believe there is also a button you can press on the controller that is linked to the microphone to activate the Overdrive feature.
I appreciate the info. I emailed Rock Band's Customer Support and they told me the same thing! It's wierd it didn't say anything about it in the instruction manual (or did I miss it?) Course, I'm not crazy about reading EVERY PAGE of the manual anyway!