The J-Hook expands your carrying capacity by allowing you to hook a smaller Travelpro bag to any of the Rollaboard cases allowing you to easily maneuver both bags with one hand. The strap is also adjustable for different size bags or to allow the smaller bag to rest on the ground for stability. This is a replacement hook. Most Travelpro FlightCrew 5 include the J-Hook. Your bag must have the female side of the J-Hook clip mounted to it for this clip to work.