I'm no expert either but, the second and third lines are the reserves aren't they? As well as the rorari and accensi, they are reserves also... The ideo being that, if the hastati don't break the enemy, the second line would, so the second line were like reserves... And the triarii were the last resort behind which the rest of the legion could form a new line if they had been broken... So the early legion you see used reserves like this. Or am i completely wrong, oh great EB ones?