Extend it as far as it will go.  Use that as a guide to cut the old pipe, allowing for the socket depth that will be lost to the overlap on the existing pipe.  Close the coupler to its retracted position.  Glue one end as you would any normal coupling.  When that end has set for a few minutes, glue the other end by applying primer and glue, then extending the coupler to slide the socket over the pipe.  It takes a little maneuvering to get the glue in the socket, but is a lot easier than digging out twenty feet of pipe to flex it into a regular coupling.  This is not the place for blue (hot) glue.  Use heavy gray as it has a longer open time.