How did the gulf of tonkin resolution expand presidential power

The Gulf of Tonkin Resolution when passed by the Congress allowed the president, LBJ, to commit American military forces as he saw fit at whatever level he saw fit to combat the Communist insurgency in Vietnam. The resolution gave the president the legal authority, or cover, to act. This act increased the president's war making powers and limited for a time the ability of Congress to force the president to allow Congressional oversight, investigation, on 
how well the president has carried out his policy. This was countered by the passing " The War Powers Act of 1973" which limits the president's war making powers. This act has never been tested in court so we do not know if it is Constitutional.