Sentence:  "When the turn arrives at the landing, the <WSD>chaser</WSD> directs the engineer where to drop the turn by hand signals. The <WSD>chaser</WSD> then unhooks the chokers, gets in the clear, and singlas to reel in the haulback line". 
Meaning of the word marked with <WSD> tags:  One who unhooks chokers from the logs at the landing.