Helaman is generous to the forces of Antipus. He allows that they were being defeated because of their forced march. Of course the Lamanites had also been on a similar forced march, though it is probably that the Nephites had to move faster than the Lamanites. In any case, the Lamanite army likely outnumbered Antipus, and the combination of the numbers and the extra fatigue was beginning to swing the tide of battle to the Lamanites.
It was at this point that Helaman arrives to press the attack on what is now the Lamanite rear. This will engage another sector of the Lamanite army, and prevent them from bringing more fresh troops to the front where the hand-to-hand combat was the most physically draining. With the need to fight on two fronts, the Lamanite superiority in numbers is now ameliorated because the reserves are also engaged.