yes overlap is when both valves are slightly open..

What I had to do to the GXP to get rid of surge @24psi&3400rpm was increase the overlap. This captures a less cyl pressure but allows more mass flow across the engine, temporarily raising the Brake Specific Air Consumption (BSAC).
Simply put I take a beating on efficiency temporarily in the rpm range but never feel it from the added boost. As soon as 3800rpm shows up, the possibility of surge is gone. I swing the cam timing back in to capture the cyl pressure and my (now crankin') turbocharger is in max efficiency, right in line with the torque peak..