Carb cap

A device that helps you take better hits by regulating the amount and direction of air that goes to the nail on a dab rig. A carb cap gives a dabber more control over their dab, much like the carb hole on the side of a hand pipe.Glass, borosilicate glass, quartz, or metal are typically the materials of choice for carb caps.

The term "carb" is shortened from "carburetor," which comes from gas engines. In an engine, the carburetor mixes fuel and air to make combustion easier. A side hole in a pipe functions as a carburetor, mixing air into the smoke path to control the heat and size of the smoke inhaled. You can also "clear" the smoke with it.
Additionally, dab rigs have adopted the carb idea. A carb cap is attached to the nail of a dab rig to temporarily block airflow to the nail and the burning concentrate.The carb cap not only regulates the amount of air, but it also helps the nail retain heat, giving the dabber more time to finish the dab. This is useful for taking extremely large dabs, which are appropriately referred to as globs.

Carb caps come in a variety of designs and all function in slightly different ways.
With directional carb caps, you have a lot of control over where the air goes.
Carb caps that are spherical or bubble-shaped work best on nails that have a beveled edge.
A variation on the bubble or spherical design are bullet carb caps.
In the nail, vortex carb caps produce a cyclone of cool air.