The modification is to mimic the passageways like the oem head gasket did. The restriction in coolant flow allows the coolant to stay in the passageway longer to extract the heat out of the cylinder/head.
Reducing the size of some of the coolant passageways between the cylinder and head forces more coolant to flow around the sides of the cylinder and enter the head near the rear of the cylinder. If the head gasket or some of the coolant passage ways did not force the water to take another path, coolant would enter the cylinder on the right side of the exhaust port and go straight up into the head outlet and return to the radiator.
Keeping the coolant in the passageways longer does not extract more heat from the head and cylinder. It reduces the amount of heat that can flow from the head and cylinder to the coolant because the coolant becomes slightly hotter and closer to the same temperature as the cylinder when the flow rate is reduced.
Heat always flows from a hot body to a cooler body. If both bodies are the same temperature heat transfer WILL NOT OCCUR. Increasing the DIFFERENCE in temperature between two bodies or substances increases the quantity of heat transferred from the hot body to the cold body. Heat is transferred by conduction, convection and radiation.
Examples:
Conduction occurs when you heat one end of a metal bar and a short time later the other end of the metal bar gets hot. Heat is conducted from one end to the other. Heat transfer by conduction travels at a speed that depends upon the coefficient of conductivity. Metals that conduct heat very well usually conduct electricity very well.
Heat transfer by radiation is occurring when you instantly feel heat being emitted from a very hot surface before the air space between you and the hot object has had time to rise. Heat transfer by radiation travels at the speed of light.
Convective heat transfer requires a hot body and some type of gas or liquid flowing over the surface to carry away heat. Convective heat transfer gets more complicated because it simultaneously involves conduction and radiation.
Convective heat transfer depends primarily upon the velocity of the medium flowing over a hot surface. Air flowing through the cooling fins of an air-cooled engine is an example of convective heat transfer. If we increase the velocity of air flowing THROUGH the fins, more heat is removed from the engine.
I probably should not go into heat transfer any deeper because it took those of us that are engineers at least two semesters of thermodynamics and one semester of fluid mechanics, to grasp and try to master some to of the concepts of heat transfer. Before we could take thermo and fluid mechanics and we had to take 5 semesters of high-level math.
I can possibly field some basic questions as long as it doesn't require some of the above prerequisites for one to be able to understand the concept I am trying to convey.