Refer to figures.
Carburetors work on the Venturi effect, which says that pressure decreases and velocity increases as a fluid is forced to go through a constriction. The Carburetor contains five critical parts, the float, the float chamber, the main discharge nozzle, the venturi, and the Throttle valve.

Because the air within the venturi is at a lower pressure than the ambient air, it creates a “Vacuum” that draws fuel out of the main discharge nozzle. The construction of the nozzle causes the fuel to form into tiny droplets, where it then vaporises in the air. Fuel is sent to the main discharge nozzle from the float chamber, which is primarily just a holding vessel for the fuel. The fuel level in the chamber is controlled by the float, a device that acts not unlike the float in a common household toilet.

Carburetor heat uses hot air drawn from the heat exchanger or heat stove (a metal plate around the exhaust manifold) to raise the temperature in the Venturi section high enough to prevent or remove any ice build-up. Because hot air is less dense than cold air, engine power will drop when Carburetor heat is used.

Anytime you apply "Carb Heat" there is a slight drop in RPMs. Warmer air is less dense than cooler air. When you apply Carb Heat you are now supplying the engine with warm air instead of cold air, thus decreasing performance slightly. Remember, a slight decrease in RPM with warm air pales in comparison to the RPM drop you will have with an iced-over Carburetor.

The direct influence by switching on the Carburettor heating on piston engine helicopters is that engine and rotor RPM decreases.

Take-off is the most important phase of flight and in this phase the Pilot must have all the available power that can be used. If carb heat is left on, the engine will lose power, reducing its RPM.

The Carburettor heating must be switched off, although Carburettor icing might be expected, during take-off.