Complex sentence: The most flexible system is the French shove coupler, in which the lower manual can slide forward and backward, so that in the backward position "dogs" attached to the upper surface of the lower manual engage the lower surface of the upper manual's keys.
Split into simpler sentences: The most flexible system is the French shove coupler, in which the lower manual slides forward and backward. In the backward position, "dogs" attached to the upper surface of the lower manual engage the lower surface of the upper manual's keys.