Sentence:  "A <WSD>chaser</WSD> follows the logs to the landing, often riding in a rigging sled hollowed out of a log, which is attached to the rear log. The <WSD>chaser</WSD> can signal to the road engineer at any point..." 
Meaning of the word marked with <WSD> tags:  Someone that follows logs out of the forest in order to signal a yarder engineer to stop them if they become fouled (also called a frogger).