The new battery (BT-DN110) contains a memory chip which provides the necessary processing power to manage the multiple shift-mapping patterns and customizable features that Synchronized Shifting delivers. By utilizing the new battery and Shimano’s E-Tube Project, riders will have access to up to three different shift modes—one manual and two customizable Synchronized or Semi-Synchronized Shift—which riders can toggle between on-the-fly via the standard EW-90A/B or EW-RS910 junction boxes.
How to access the new features:
First ensure you have the new BT-DN110 battery, which is the only battery that allows you to take advantage of automated front shifts (i.e. full Synchronized Shift mode), or automated rear 'correction shifts' (i.e. semi-Synchronized Shift mode).
Next, if you are a current Di2 user with compatible front (FD-6870/FD-9070/FD-9150) or rear derailleurs (RD-6870/RD-9070/RD-9150) you can download the latest E-TUBE firmware either by a cable connection to a PC, or via Bluetooth from your tablet or smart phone to your components.
Changing modes:
Each mode is customizable in E-TUBE, meaning you can choose different combinations and customized shift patterns if you wish (for example, two semi-Synchronized Shift modes offering different ‘correction’ shifts, or two full Synchronized Shift modes that assign a front derailleur command on differing chainrings).
For more information: A helpful video explains the benefits of E-TUBE and the potential shift mapping options here: http://e-tubeproject.shimano.com/about/semisyncroshift.html
What is Synchronized Shifting?
A function which allows riders to concentrate on only up or down shifting the rear derailleur while the Di2 system automatically picks the ideal gear combination from both the front and the rear derailleur. Even in Synchronized Shift mode riders still have full control over the front derailleur. Synchronized Shifting is designed to complement rather than replace manual shifting, and is especially useful when you are using auxiliary sprint, climbing or TT shifters, or, for Dura-Ace, the shifters on the top of the hoods.
With one press of the rear shifting lever, the function can achieve the optimal front/rear gear for your personal riding style. The E-TUBE PROJECT app or website can be used to pre-set exactly when changes take place.

Shifting the rear derailleur from 28T towards 11T, the system recognizes the preferred gear ratio step should be made between the 15T and 17T sprocket.

Shifting the rear derailleur from 11T towards 28T, the system recognizes the preferred gear ratio step should be made between the 25T and 21T sprocket.
What is Semi-Synchronized Shifting?
Rather than automatically shifting the front derailleur, semi-Synchronized Shifting mode automatically shifts the rear derailleur when the front derailleur is shifted. This allows riders to obtain optimal gear transitions for smoother riding.
Again, the E-TUBE PROJECT app or website can be used to determine exactly when this change takes place.
What is Manual Mode?
Manual mode is the system all DURA-ACE Di2 and ULTEGRA Di2 riders had access to until this point. Manual mode provides an automatic trim adjustment on the front derailleur but there is no corresponding gear shift of the front derailleur based on the shift action of the rear derailleur, or vice versa.