This is the 10' long elastic shock cord (or bungee cord) that recoils the chain on recovery.
Concept2 Indoor Rower GENUINE New Bungee / Shock Cord for Model C, D, E machines
Concept2 RowErg Rower Shock Cord 1731 x2
Rowing Machine Elastic Rope Bungee Shock Cord for Training Fitness Equipment
