Complex sentence: In a traveling-screw linear actuator, the motor "crawls" up and down a lead screw that is restrained from spinning--the only spinning parts are inside the motor, and may not even be visible from the outside.
Split into simpler sentences: In a traveling-screw linear actuator, the motor "crawls" up and down a lead screw that is restrained from spinning. The only spinning parts are inside the motor, and may not be visible from the outside.