I'll take a poke at it but someone please correct me if I am wrong. Adjustable cam gears change when the valves open and close in relation to combustion and piston position. This change in valve timing can yeild more horsepower because it changes when and how much air/fuel exits or enters the cylinders. Vtec controllers on the other hand don't directly control valve timing but they control when the ECU switches from low-throttle settings to high-throttle settings. Cam gears would still affect both settings throughout the entire RPM range.