BK2 series brakes are designed to fit BMP, BMR and BMSY series motors, they are used to lock a motor when not in use.
These are hydraulic wet disc brakes, unlike an automotive-style brake, require pressure to release rather than pressure to apply the brake, when no system pressure is present, they apply a minimum if 410Nm of braking force.
They are mounted to the front of a motor, and along with either a counterbalance or shuttle valve (which is generally mounted to the motor also) create an individual braking system for each motor in a circuit. They are designed as a static park brake only and not intended for dynamic (typical driving type) braking.