Reliable and proven Di2 technology makes it way to Ultegra for incredibly easy and very precise shifting. Di2 shifters send a signal through wires to small motors built into the derailleurs so there is no straining to pull a cable or relying on spring tension. All the buttons are in the same location as the standard STI levers so the learning curve is virtually nonexistent. The buttons require a light touch and only 2mm of movement for perfect shifts every time. Di2 shifters are as soft as perhaps a mouse click and can be a huge benefit for anyone with hand issues like arthritis or carpal tunnel.
Because the levers don't need to house any shifting mechanism due to the Di2 design they can be made as light and ergonomic as possible. They have a reach adjustment to fit all hand sizes. The Di2 6770 STI levers are, in fact, about 135 grams lighter than the 6700 levers. The levers are more sleek with a lower profile and have improved ergonomics designed to enhance long-distance comfort. Shifting can be done while coasting with the derailleur executing the shifts when pedaling resumes.
The Ultegra Di2 STI levers are one part of the Di2 system including shifters, derailleurs, battery, battery charger, and several cables/wires. The wiring kits are simplified and the Ultegra Di2 components are not backward compatible with first generation Dura-Ace Wiring kits. However, the second generation Dura-Ace Di2 will share the same improved wiring kits as Ultegra Di2.