It is not always possible to unequivocally determine the direction of intensity change, such as is for example the case for highly overlapping signals next to each other and of which the intensity changes in the opposite direction. This is where the off diagonal peaks in the synchronous 2D spectrum are used for:
#if there is a positive cross-peak at (x, y) in the synchronous 2D spectrum, the intensity of the signals at x and y changes in the same direction
#if there is a negative cross-peak at (x, y) in the synchronous 2D spectrum, the intensity of the signals at x and y changes in the opposite direction
As can be seen in the 2D synchronous spectrum on the right, the intensity changes of the peaks at 10 and 30 are related and the intensity of the peak at 10 and 30 changes in the opposite direction (negative cross-peak at (10,30)). The same is true for the peaks at 20 and 40.