A knuckle is the part of the coupler assembly that transmits the load between two freight cars. When a train is stretched, the knuckle carries 100% of the draft load between two cars.
During train operation the knuckle is held closed by the lock, which fills the space between the knuckle’s locking face and the knuckle tail wall inside of the coupler. When the coupler is operated, the lock is lifted out of the way by the lock lifter. The lock leg then engages the thrower, which pushes the knuckle open.
See it in action: