Clasp-knife response refers to a Golgi tendon reflex with a rapid decrease in resistance when attempting to flex a joint, usually during a neurological examination.
It gets its name from the resemblance between the motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied.
However, if sufficient force is applied, limb resistance suddenly decreases, presumably mediated by the Golgi tendon reflex (also call autogenic inhibition).
More recent work strongly suggests that tendon organs are not involved in the clasp knife reflex, but that other sensory receptors in muscles are responsible.
[2] Passive flexion of elbow meets immediate resistance due to stretch reflex in the triceps muscle.