SRAM's chain spotter included with the 2012 Red Yaw front derailleur works so effectively SRAM decided to offer it separately. The SRAM Chain Spotter keeps the chain from falling in toward the frame for worry-free shifting. It virtually takes the dropped chain off the table. Think of it as "frame insurance" for a valuable carbon frame. The SRAM chain spotter is nicely made and can be adjusted independently of the front derailleur.
The Chain Spotter will only fit SRAM Red 2012, Red 2011, SRAM Force, SRAM Rival, and SRAM Apex Braze-on front derailleurs. It will not fit SRAM Red 2011, SRAM Force, SRAM Rival, SRAM Apex Clamp-On FD's. Another tech note - you must use the bottom mounting hole on the Red 2012 Clamp-On mounting clamp when using with Red 2011, Force, Rival and Apex Braze-on front derailleurs.