The pulley is held in with a stainless bolt and nylock nut. The barrel adjuster is removed and then threaded into one end of the aluminum body. At about a 70 degree angle from the adjuster is the exit for the cable. This end then butts onto the final piece, a stainless steel nipple that threads into the derailleur in place of the barrel adjuster. Describing in words is more complex than it is, a pulley that eliminates the bend that the cable housing has to make to reach the the rear derailleur barrel adjuster.
Installation is simple.
Shift to the smallest cog and unclamp the cable from the derailleur and pull it out of the derailleur and rear housing. You will need to trim the housing for the new shorter, straight run. Unscrew barrel adjuster from the derailleur and install it into the threaded end of the Rollamajig. Take the stainless nipple and thread it into the derailleur. Thread the cable back through the housing then in through the barrel adjuster on the Rollamajig.