There is an error in the equation in the Technical Reference Manual. The correct equation should be


This equation then does match the words as shown by this simple example. Let's assume we have 5 modes a through e, where b,c, and d are grouped. The words say that R = [a^2+|b+c+d|^2+e^2]^.5 which can also be written
R = [a^2+b^2+c^2+d^2+e^2+2bc+2bd+2cd]^.5

This is exactly what you will get by expanding the equation given above.