Defining relation of a fold over design

Dear All,

I am reading the book DC Montgomery on my own. The book says that the defining relation for a foldover design will have L + U - 1 words used as generators where L = number of words of like sign, U = number of words of unlike sign. Here U-1 = number of independent products of words of unlike sign. Can some one sketch the proof of this for me ?

Best Regards.