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.