The heat riser is part of the fast warm up system on a stock vehicle. The bimetalic heat sensor coil will force the butterfly at the rear of the passenger exhaust manifold to close, forcing the exhaust to flow from the passenger side of the engine up through the passage that is cast into the intake manifold and then out the drives side of the exhaust. On dual exhaust cars it frequently caused the passenger side to rust out very quickly when the valve would stick shut and that side was not brought up to full operating temperature and the water vapor rotted the one side out very quickly. Less of a problem on single exhaust cars.
The trick is to wire it open, or remove the butterfly, or replace it with the plain cast iron spacer used on fuel injection cars. The valves are very troublesome in many cases. If you don't drive much in cold weather and do not mind the extended warmup, this is a good modification. If you have headers you obviously do not have the heat riser butterfly valve anymore so you may want to block the heat riser passages between the heads and the intake. This is a good mod if you run an aluminum intake. With a cast iron intake it causes real long warm ups so maybe a restrictor would be better than a fully blocked passage.