Sentence:  A one-way <WSD>sprag</WSD> clutch (figure 40-36) consists of a hub and a drum separated by figure-eight-shaped metal pieces called <WSD>sprags</WSD>. The <WSD>sprags</WSD> are shaped this way so they can lock between the races when a race is turned in one direction only. 
Meaning of the word marked with <WSD> tags:  A billet of wood; a piece of timber, a similar solid object or constructed unit used as a prop.