Extrude honing is a sort of "port and polish" that forces a "putty" through the passages that cuts and smooths out the walls slightly. Its main advantage is that since it is a liquid with cutting particles and such in it, it floiws like air, therefore giving the passages more laminar(sp?) flow. So technically it does open up the plenums more, but the focus on using extrude honing is rising the flow without extremely opening up the passages, especially in difficult manifolds like ours.
