Sentence:  The shaft on which the <WSD>percussor</WSD> is fastened is so adjusted that the spring, while expending its force, throws the hammer a little beyond the line of centre, so that the blow is struck lightly and quickly, and the hammer rebounds very much as it strikes and rebounds from the string of a piano when a note is struck on the keyboard. 
Meaning of the word marked with <WSD> tags:  A small hammer-like tool that a physician uses to provide a light blow to a body part.