Different Shifting Modes for Shimano Di2

You will be able to change between different shifting modes for the Di2 after you have paired it with your Karoo or Karoo 2. You will find these settings under the sensor inside the "Sensors" app after you have paired it.

There are 3 different modes to choose from:

S1
With Full SYNCHRONIZED SHIFT mode enabled, you can control both derailleurs with a single shifter. The front derailleur detects the position of the rear derailleur and automatically shifts to the most efficient gear for the optimum chainline, so you never have to worry about front or correction shifting.

S2
Semi-SYNCHRONIZED SHIFT mode is a function that automatically shifts the rear derailleur when the front derailleur is shifted, in order to obtain an optimal gear transition. This gives you more control to match the gear combination to your personal riding style. SHIMANO SYNCHRONIZED SHIFT is completely compatible and customizable with E-TUBE.

M
This mode gives you full control over your gear shifting. It is fully Manual.

Also see: Shimano Di2 integration

Check out Shimano's Video on Syncro Shift here:

Source: https://bike.shimano.com/en-EU/technologies/component/details/synchronized-shift.html