There are two modes (in addition to standard manual)...and hopefully the names just reflect Japanese terminology that doesn't translate well:

1) "Semi-synchro"-a delightfully uninformative and confusing name name..... In this mode, you manually trigger the FD shift...BUT it has a multi-shift of 2-3 gears attached to to the FD command with no extra button pushing....thus being roughly equivalent to the Campag simultaneous gear dump (just with one shifter command)...getting you to a same or sequentially lower or higher gear on your other chainring.

2) "Full synchro"-another WTF does that mean name. In this mode, you setup preconfigured shift points as you go up/down the cassette for the FD to trigger (with the option to do a multi-shift to compensate and get sequential gearing). With this mode you never need to command the FD at all, in fact you don't even need FD buttons. Just tap up or down your cassette, and at your pre-configured points the FD will on its own shift and the rear multishift.




How does this actually work in reality, with an actual bike transmission??

Example...my Di2 gravel/touring rig has a 46/30 crank with an 11-32 cassette (11-12-13-14-16-18-20-22-25-28-32). I have the R/L brifters setup for paddle-shifting (left is always an easier gear, right is a harder gear. Rear buttons are RD shifts front buttons are FD shifts)

1) I have "semi-synchro" setup for 3 multishifts whenever the FD is triggered. FD is triggered, the RD will shift 3 gears in the opposite direction to compensate-getting me (more often than not) a next step higher/lower gear. This mode I like when loaded down with panniers for commuting-since I live more in my granny ring when loaded down. So I'm bombing down a hill in my 46-14 and hit a hill, I can either multishift regularly down the cassette, or trigger the FD which will land me in a 30-11.

2) I have "full synchro" setup so I have 15 or 16 sequential gears (can't remember offhand exactly) in my 22 gear drive train. Whenever I get downgeared in the big-dog to 46-28, the next shift down gearing triggers the FD and a 3Xmultishift getting me to 30-20. When upshifting on my granny gear once I get to 30-16 the FD/multishift is triggered to get me the next gear up my 46-22. This mode works nicely for unloaded riding around.


The nice thing about Di2. It is digital and all basically mouse buttons. Within Shimano's framework you can make it however you want and as contrived/specious/WTF/insane as you want....or if you liked it better the old way-there's always standard manual mode.