The Oceanus Shock Cord is an alternate attachment to the standard rope extension, and comes with a webbing attachment. The webbing attachment affixes to your board fin-screw area, then you can attach either the standard rope extension or the shock cord. The Shock Cord is shorter and has elastic bungee.
Having the elastic bungee can help absorb the "shock" and load of the reel going out all the way quickly. Big waves or strong currents can also pull you in one direction and your board in another, so having the extra rope or Shock cord to ease the tension is necessary to keep the reel performing well time after time.