The Shimano Ultegra RD-6870-GS is an electronic Di2 rear derailleur designed for 11-speed road bikes, ideal for riders building or upgrading Shimano Ultegra 6870 Di2 groupsets who need a wider gear range than short cage versions. Its medium (GS) cage supports low sprockets from 28T to 32T, a maximum front difference of 16T, and total capacity of 37T, making it suitable for compact cranksets paired with cassettes like 11-32T on varied terrain such as rolling roads or light climbing. It mounts directly to the frame hanger and connects via E-tube wiring to Di2 shifters like ST-6870, enabling programmable multi-shift speeds (slow to very fast) and crash protection that disengages the motor on impact to prevent damage. Without a clutch, it relies on precise electronic servo shifting for clean performance on smooth roads, but riders should inspect chain tension on rougher surfaces.