KEB servo brakes are popular because they have low inertia, offer low backlash braking, and can operate at high speeds. This video describes how KEB spring-set servo motor brakes work.
When an electromagnetic coil is powered with DC voltage, a magnetic circuit is created. The magnetic field is strong enough to pull an armature into the magnet housing and compress multiple embedded springs. With the springs compressed the friction lining, hub, and shaft are free to rotate. When power to the magnet is released the springs push against the armature. The friction lining is sandwiched between the armature and a secondary friction plate. Because the friction lining is connected to the shaft with a splined connection the shaft stops rotating and is held in place.