For what it's worth...
In the real world, using heat sink could be a material or tool.
A heat sink is a piece of metal, usually aluminum, to help radiate heat from a component to add in cooling. If anyone remembers the big fins from their car radios there were big heat sinks on the backs for the power transistors. Now more commonly seen on computer's CPU.
Not sure the context in the Yamaha manual, but when you used a heat sink, you also used heat sink compound
. A material put between the part getting hot and the other piece to ensure surface area contact and to aid in transferring heat. Maybe Yamaha wants the compound to help the carb
heater heat the carb?
As for a tool, when soldering you may put a temporary heat sink on an item you are soldering. If you have tiny part that is being soldered but is sensitive to heat, you would clamp something on the main
component (sometimes a vice grip..) to carry heat away as you soldered the terminals.