The carbon brush holder in a car's regulator is a crucial component that primarily functions by applying pressure through springs onto the carbon brushes, which slide in contact with the commutator or collector ring surface. This ensures stable current transmission between the stationary and rotating parts. For automotive motors, both the carbon brush holder and the carbon brushes are vital components that work together to maintain the motor's normal operation and smooth current transmission.

Please note that different types of cars and motors may have carbon brush holders with varying designs and structures, so there may be some differences in practical applications. For more detailed or specific information, it is recommended to refer to relevant automotive or motor technical manuals or consult professional automotive maintenance and technical personnel.