Shimano's new Dura-Ace Di2 RD-R9150 11-Speed Rear Derailleur boasts a sleek profile that practically hides from the wind and finally brings the Synchronized Shift functionality honed in the trail-focused XTR Di2 system onto the road. Enabling Synchronized Shift lets the Di2 system work its magic to automatically shuttle your chain from cog to cog without any input from you and intelligently manage the fleet of cogs at the aft-end of your drivetrain. That said, it isn't a requirement to use, so the traditionalists among us will breathe a sigh of relief that this derailleur also allows the same precise, manually controlled and customizable shifting we've come to expect from Di2.
By putting the rear and front derailleurs in communication with each other, Synchronized Shift can actually calculate your efficiency and perform simultaneous front and rear shifts to eliminate cross-chaining and set you up in the best chain line for any given gearing. These shifting patterns depend on the direction you're shifting and the combination you're shifting into, and they're also programmable via Shimano's E-Tube system, so you can set your preferences in advance to ensure that your internally communicating drivetrain isn't conspiring against you.
The shifting itself is peerless, as the derailleur always knows where it is, so its travel isn't restricted or lengthened by imperfect cables. It situates the chain squarely on every cog and it jumps between those positions with startling speed. As with other Shimano Di2 bits, the Dura-Ace rear derailleur is compatible with Shimano's rust-, stretch-, freeze-, and generally worry-free E-Tube wiring system. Its modularity makes it easy to set up on virtually any frame, though it does cohabitate with less friction when the frame is built with Di2 in mind.