When you first jump on the throttle from idle the VTC's immediatly adjust your timing to put you in the meat of your power band. They continue to adjust through your midrange and return to normal around 4500 RPM's I believe.
I too am still learning exactly when they are active and what they do, but I have been told multiple times it can have a huge effect on your low-end. My guess is that my VTC selenoids are shot and no longer functioning.