The Heat Riser Valve is part of the engine warm up system. When the engine is cold, a spring on the heat riser valve holds it closed, forcing exhaust gases from the right-hand side of the engine to pass through passages in the head to a plenum riveted to the underside of the intake manifold. This quickly warms the manifold, delivering better fuel atomisation under cold conditions.
When the spring on the hear riser gets hot it loses tension, allowing a weight attached to the valve pivot to open the valve. There is a risk that the valve could get stuck closed (due to rust for example), so many Corvette owners wire this valve open or remove it completely.
Fuel Injected cars didn't require the heat riser valve, they used a spacer to fill the gap between the manifold and the exhaust created when the valve is removed. Rather than wire the valve open, I fitted this spacer to the Red Rocket. It's a small thing, but it cleans up that side of the engine a little, removes an obstruction from the exhaust and avoids a potential point of failure.