Makes bad chains good.
While Shimano's XTR group has always been recognized as worthy of its top-shelf status, the smooth action of XTR rear derailleurs is a big part of the legend. This XTR Shadow Rear Derailleur RD-M980 is a second generation Shadow XTR unit. As such, it benefits from the shadow design in the same way as the 970 did. The low-profile geometry keeps it closer to the dropout and safer from impacts with roots and rocks.
Aside from that, there are a few significant changes for this model. Both pulleys now have sealed bearings, and the pulley cage blends a carbon fiber outer with an aluminum inner plate for a just-right combination of lightweight, strength, and stiffness. Shimano engineers also changed the return spring so that it works better with full-length cable housing. With the new spring, the XTR 980 Rear Derailleur snaps onto smaller cogs as quickly as you can make the shifts.
You'll have the choice of a medium cage (GS) or long cage (SGS). The differing lengths of the cages translate into different chainwrap capacities for each version. The GS derailleur is best paired with a 2x10 XTR Crankset. The long cage SGS model has a greater chainwrap capacity, so it will handle the XTR Trail Triple Crank.
The Shimano XTR Shadow Rear Derailleur RD-M980 has a top normal spring configuration, which means that it returns to the top gear when the cable tension is released (which is normal). It's designed to work in harmony with Shimano Dyna-Sys 10-speed Rapidfire shifters and 10-speed cassettes with a maximum 36t large cog.