Reliable and proven Di2 technology makes it way to Ultegra for incredibly easy and very precise shifting. The best thing about Ultegra Di2 is you get Dura-Ace Di2 performance at about half the price.
Most derailleurs use a spring and cable tension to move the chain from cog to cog. The Shimano Ultegra RD-6770 Di2 rear derailleur employs highly innovative digital technology where an electronic signal is sent to a worm drive motor move the chain from gear to gear. Shifts are lightning-quick with no hesitation. Performance is on par with Dura-Ace Di2 except the Ultegra uses a slightly bulkier motor and has an alloy cage instead of carbon.
For those concerned with damaging the derailleur in a crash, Shimano engineers included a servo-saving mechanism that allows the rear derailleur to handle a big side impact with minimal damage. The derailleur can also be fine-tuned in adjustment mode (by holding the button on the control module near the shifters until it turns red) with a few quick presses of the shifters to exactly line up the derailleur to one of the cogs.
The Ultegra Di2 rear derailleur is one part of the Di2 system including shifters, derailleurs, battery, battery charger, and several cables/wires. The Ultegra Di2 rear derailleur is not compatible with Dura-Ace Di2 shifters or first generation cables.