The heat riser passes coolant right under the intake manifold and moderates the temperature of the intake manifold and carb. It really is functionally the same purpose as the flap in the OEM exhaust manifold; at cold temps send additional heat to the intake/carb and at hot temps keep additional heat from the intake/carb - it just does this using the engines cooling system.
With a hot truck, the coolant draws heat from the intake manifold and carb which helps keep the fuel from turning to vapor. I had vapor lock on the 40s only run when I was not using the fluid heat riser, since installing it I have not had a any vapor lock.
With cold starts, as the coolant warms up, the heat is transferred to the intake manifold and carb which helps the fuel and air atomize more effectively. Cold fuel does not like to mix with air as well as warm fuel.
Ricardo Maestas - Bio Page
1999 LX 470, 1971 FJ-40