The bringing the ph up to 7.8 is for trying to get the metals to stain the filter, he wants you to put a trichlor puck in the skimmer basket - this way when the high ph water hits the chorine puck, it makes the metal precipitate out of the water and land on the filter.
I understand this to mean that the high chlorine in the puck makes the metal precipitate and stain the DE in the filter, The backwashing would then flush the DE and the metal along with it, removing it from the pool.

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