Most derailleurs use a spring and cable tension to move the chain from cog to cog. The Shimano 7970 Di2 employs highly innovative digital technology. A worm drive motor move the chain from gear to gear with 21st century speed and accuracy. After riding with the Di2 rear derailleur, we found it to be lightning-quick with no hesitation.
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 with a few quick presses of the shifters.
The Di2 rear derailleur is made with a carbon fiber cage and 11-tooth sealed-bearing pulleys. A titanium fixing bolt with alloy and titanium hardware allow the weight to be only 222 grams. The Dura-Ace Di2 rear derailleur is one part of the Di2 system including shifters, derailleurs, battery, battery charger, and several cables/wires.