Shimano's electric-shifting 11-speed Ultegra Di2 Rear Derailleur is driven by battery power and a servo motor, so shifts are fast, precise, and perfect every time. It's made of light, tough cold-forged aluminum while mid-cage and short-cage models give you your choice of cassette. Since there aren't any cables or housing to stretch or fail, maintenance is dramatically reduced and shifting accuracy is unprecedented. This high-tech derailleur is even designed to move inward under impact to reduce the potential for damage in a crash.
An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 44/29 chainrings and a 11-28 cassette: (44 - 29 = 15) + (28 - 11 = 17) = 32-tooth chain-wrap capacity.