Basically making the ports on the intake and exhaust bigger, letting more air in and fuel (if you upgrade the fuel system) and polishing smoothes the porting even more and makes the the air flow smoother. Remember more air and fuel in the combustion chamber: mo powa.
This is porting/polishing. This is the head for my newly accuired 97 Ranger 2.3 5 speed...this is the "late" dual plug head...a bit different than the Mustang style dual plug head.