Originally Posted by
mtseymour
Even in full synchro mode, the computer doesn't take over. Depending on the terrain, I might downshift from the 50 to 34T sooner than the programmed shift point. I might also simultaneously downshift the front derailleur, and upshift one or two gears at the back to get a smooth transition. Unlike eTap, Di2 is able to shift front and rear at the same time.
The full synchro mode also acts prevent a severe cross-chain when I downshift too far in the rear (eg. from 28 to 30T) while in the big ring. This is especially useful in rolling terrain with abrupt transitions to steep climbs.
Yeah, I know - poor choice of words. I did have an experience where it was fighting me though. I went to make a double shift in anticipation of some change in terrain (can't remember which) and it shifted the rear back to where it wanted it not where I wanted it. Actually, I'm not sure if I was in semi-synchro or full at that point. I did my last ride in semi and it seemed a little better/smoother and more "controlled" I guess - although it did let me cross chain big ring to big cog like you said full would prevent. I'll keep playing with things and keep an open mind.