From Avatar the Legend of Korra :

The Metalbending Police Force is the elite law enforcement organization of Republic City, founded by Toph Beifong. By using metalbending to manipulate the metal cables within the cylindrical spool devices on their backs, the officers are able to restrain criminals and navigate the city quickly and efficiently. Along with this equipment, they are also known to carry truncheons and riot shields. The Metalbending Police Force monitors events in the city through the use of several airships, and this mobility allows them to easily apprehend felons and deploy forces for pursuit.