What type of brush allows the greatest airflow to the hair for quick drying while directing it into design lines?

The vent brush is specifically designed to allow maximum airflow to the hair while styling or drying. Its structure typically features widely spaced bristles and an open back, which not only facilitates efficient air circulation but also helps to direct the hair into specific design lines effectively. This combination makes it particularly valuable for achieving a quick drying time while maintaining control over the hair's shape and style.

In contrast, round brushes are primarily used for curling and adding volume but may not provide the same level of airflow as a vent brush. Cushion brushes are designed for smoothing and detangling rather than drying, as they have a padded base that does not allow for significant airflow. Flat brushes, while good for straightening hair, also do not offer the same efficiency in drying compared to a vent brush. Thus, the vent brush stands out as the optimal choice for quick drying and precise styling.