Carb caps are like carburetor for your oil rigs. Carb Caps restrict the amount of airflow to your nail, allowing you to collect the final bits of your oil. By capping the nail, you restrict the airflow into the rig through the small contact area between the carb cap and the edge of the nail. Further inhaling strongly the reduced airflow places the entire rig under low pressure, including the oil that pooled on the nail without vaporizing. This enables the concentrate to boil at a much lower temperature, giving you a bloom of thicker and better tasting smoke. Carb cap actually allows you to finish the dab, instead of wasting it as a puddle.