I have this very clever formula provided by nflsales which does a perfect job

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I've spent hours trying to disect it and understand it. I understand the basic concepts of ROWS, COLUMN and SUMPRODUCT but I'm really struggling to understand how it is all working together and I wondered whether anyone would be kind enough to break it into small pieces and translate it into plain English so I can get a better understanding of what the elements are doing, especially in the context of two arrays MMULT is using. The more basic the explanation the better.

Many thanks