From road.cc
“The front derailleur reads the position of the rear derailleur and automatically operates the front shift to position the gears in the most efficient gear and best chain line so the rider never has to worry about front shifting and correction shifts again. Synchro Shift… improves efficiency of shifting and riding.”
Essentially, you just decide whether you want to change up or down, and the Di2 system does the rest automatically. You don’t need to worry about the chainring/sprocket combination, that is all decided for you. If you want to go one gear harder and the most efficient way of achieving that is for you to be in a bigger chainring, the Di2 will move you there.
I’m thinking; 1×11 means that I don’t have to worry about ‘correction shifts’ but hey-ho, I do like a bit of tech, whether its relevant to me (right now) or not.
edit: also on the STW News page