overrun brake
brake system common on small trailers, where the motion of the trailer with respect to the towing vehicle is used to actuate the brake
en-US/ˌoʊvɚˈɹʌnv ˈbɹeɪk/
en-GB/ˌəʊvəˈɹʌn ˈbɹeɪk/
brake system common on small trailers, where the motion of the trailer with respect to the towing vehicle is used to actuate the brake