The sum of the squares of the deviations of the predicted values from
the mean value of a response variable, in a standard regression model.
https://en.wikipedia.org/wiki/Explained_sum_of_squares
SSreg = ∑(ŷᵢ - ȳ)²
When a constant is fit to the regression, the average of y = average of ŷ.
In the case where the constant is not fit, we use the sum of squares of the predicted value
SSreg = ∑ŷᵢ²