The blocking plate is part of a prosthesis system. Combined with the blocking hook it builds a unit for the knee joint.
This assembly is used to prevent accidental variable knee flexion as a protection of the knee joint. In this case the rotatable blocking hook snaps in on the blocking plate in the extended position and blocks the flexion of the prosthesis.
When a lifting device is lifting the blocking hook in the extended position, the flexion of the knee prosthesis can be activated. This enables for example, that the patient can sit down.