Torque Reduction covers a lot when talking about DSG's.

Are you asking about the max torque limit? Or asking how the DSG tells the ECU to do a reduction?

Do you have a DSG functionsrahmen? How the DSG does an upshift is very hard to understand, honestly the FR doesn't make it easier but if you have logs too it starts to make more sense.

In the attached picture the "Warten Synchron" is the section you are going to want to understand.  There are maps that control how long that phase lasts, and from there an engine speed gradient to match the speed of change to the time is calculated.  Then amount of torque required to match that gradient is calculated.  Finally the DSG adds the that torque and the amount of torque the engine is making together, if that torque value is higher then the max clutch torque then the extra is requested as a torque reduction.