The term 'Sudden Jerk' is a phrase used to describe an abrupt, unexpected movement. This movement can be described by the crossword solutions of START (5 letters), TWITCH (6 letters), YANK (4 letters), and SPASM (5 letters). A START is a sudden, involuntary movement, usually caused by surprise or fear. A TWITCH is a small, brief, and rapid movement of a muscle. A YANK is a sharp, sudden pull or jerk. Finally, a SPASM is an involuntary contraction of a muscle or group of muscles. All of these words describe the same type of sudden, jerky movement.