What is the meaning of "clasp-knife spasticity"?

Involuntary tightness: This occurs with certain types of strokes, and with spinal cord injuries. Involuntary tightening of the flexor muscles, with intermittent gear as you try to passively straighten a muscle out or extended from the flex position.

Spastic tone: In patients with spasticity the arms and legs are stiff and move with difficulty. A very specific form of stiffness will cause a form of release during movement which is similar to the sudden release when a clasp knife releases. The tone can be very tight, then releases, then tightens again. This is the classic findings in spastic cerebral palsy.