If memory serves, McPhail did not overturn the decision that the bal was illegal. By baseball rules, in fact, the bat was illegal because the pine tar extended too high up the barrel. He overturned the decision by evoking "the spirit of the rule" perspective that there was no advantage to the batter if pine tar was not within the rule's boundary. Essentially what he ruled was that the pine tar did not assist Brett in hitting the home run.