Nucleophilic attack by the 3' OH on the alpha phosphate releases pyrophosphate, which is then subsequently hydrolyzed (by inorganic phosphatase) into two phosphates. This hydrolysis drives DNA synthesis to completion.
The 3' OH's nucleophilic attack on the alpha phosphate leads to the formation of pyrophosphate, which is subsequently broken down by inorganic phosphatase into two phosphates. This breakdown supports the completion of DNA synthesis.