The Collaborative International Dictionary of English v.0.48:

Link motion \Link" mo"tion\ (l[i^][ng]k" m[=o]"sh[u^]n) n.
   (Steam Engine)
   A valve gear, consisting of two eccentrics with their rods,
   giving motion to a slide valve by an adjustable connecting
   bar, called the link, in such a way that the motion of the
   engine can be reversed, or the cut-off varied, at will; --
   used very generally in locomotives and marine engines.
   [1913 Webster]

   Note: The illustration shows a link motion for a vertical
         engine, c representing the shaft carrying two
         eccentrics, a and b, for making the engine run forward
         and backward, respectively, their rods e and d being
         jointed to opposite ends of the slotted link f, in the
         opening of which is a pin g which is attached to the
         valve rod h. The valve will receive the motion of the
         forward eccentric when the link is in the position
         shown, and the motion of the backward eccentric when
         the link is shifted so far to the right as to bring e
         in line with h, or a compound motion derived from both
         eccentrics when the link is shifted to intermediate
         positions, the compound motion causing the valve to cut
         off the steam at a point determined by the position to
         which the link may have been shifted.
         [1913 Webster]