SL brake is a hydro-mechanical device which is Spring Loaded and hydraulically released.
A spring load exerts a compressive force between steel plates, which are mounted on the rotating shaft, and sintered plates mounted in the gear tooth ring.
When oil pressure is applied, the spring load is relieved and the plates are free to move axially on the shaft hub allowing free to move axially on the shaft hub allowing free rotation of the shaft.
The brake is re-applied by releasing the oil pressure.