Helaman used a variant of the tactic of luring the Lamanites away from their fortified city. In this case, it was supposed that it was most of Helaman’s army that contended with the Lamanites, and when the engagement began, Helaman had his men retreat as though fleeing from the greater numbers of Lamanites. In the heat of the battle, the Lamanites would have seen themselves as winning the battle, and therefore pursued the Nephites to finish them.
That pursuit led them past the secreted forces of Gid and Teomner, and thus the Lamanites inadvertently allowed Nephite soldiers to be at their rear.