Synchronized Shifting is a huge subject, and if you'd like to know more about that I recommend reading my complete guide to synchronized shifting.
Short on time? Here is a quick summary.
Modern Di2 bikes have three different shift modes: manual mode, shift mode 1 (usually semi-synchro), and shift mode 2 (full synchro).
In semi-synchronized mode the rear derailleur will automatically shift to compensate for front derailleur shifts.
In full synchronized mode, the front derailleur shifts automatically as the rear derailleur reaches certain gears. This is great on TT/Tri bikes.
Change selected shift mode
While you can use the E-Tube Project apps to change what each shift mode does, that doesn't actually change the selected shift mode on the bike.
To change the bike to the next shift mode, press the function button twice. The LED will then indicate the selected shift mode.
On 11-speed bikes, the button and LED are on the main junction - near the charge port.
This is what the LEDs mean:
- Solid red/green lights: manual shift mode
- Twice blinking red/green lights: shift mode 1 (default: semi-synchronized shift)
- Three-times blinking red/green lights: shift mode 2 (default: synchronized shift)
Read more on Synchronized Shifting in the complete guide to synchronized shifting.