The constants used in the power interpolation

I am interested in knowing more about the precision of the measurements that we are doing using the emonlibCM library but studying the code I cannot understand the constants used in the power interpolation. Specifically the variables:

y [i] = sin (phase_shift) / sin (sampleRate);
x [i] = cos (phase_shift) - y [i] * cos (sampleRate);

Can you help me with this part?

Best regards

Look at the references that are given in the post from where library is downloaded.