8 Lastly, “The sons of Ephrem bending and shooting bows, have been turned back in the day of war”. Following after the law of righteousness, unto the law of righteousness they have not attained. Why? Because they were not of faith. For they were that generation whereof the spirit has not been trusted with God: but they were, so to speak, of works: because they did not, as they bended and shot their bows (which are outward actions, as of the works of the law), so guide their heart also, wherein the just man does live by faith, which works by love; whereby men cleave to God, who works in man both to will and work according to good will. For what else is bending the bow and shooting, and turning back in the day of war, but heeding and purposing in the day of hearing, and deserting in the day of temptation; flourishing arms, so to speak, beforehand, and at the hour of the action refusing to fight?
But whereas he says, “bending and shooting bows,” when it would seem that he ought to have said, bending bows and shooting arrows....Some Greek copies to be sure are said to have “bending and shooting with bows,” so that without doubt we ought to understand arrows. But whereas by the sons of Ephrem he has willed that there be understood the whole of that embittering generation, it is an expression signifying the whole by a part. And perhaps this part was chosen whereby to signify the whole, because from these men especially some good thing was to have been expected....Although set at the left hand by his father as being the younger, Jacob nevertheless blessed with his right hand, and preferred him before his elder brother with a benediction of hidden meaning....For there was being figured how they were to be last that were first, and first were to be they that were last, through the Saviour's coming, concerning whom has been said, “He that is coming after me was made before me.” In like manner righteous Abel was preferred before the elder brother; so to Ismael Isaac; so to Esau, though born before him, his twin brother Jacob; so also Phares himself preceded even in birth his twin brother, who had first thrust a hand out of the womb, and had begun to be born: so David was preferred before his elder brother: and as the reason why all these parables and others like them preceded, not only of words but also of deeds, in like manner to the people of the Jews was preferred the Christian people, for redeeming the which as Abel by Cain, so by the Jews was slain Christ. This thing was prefigured even when Jacob stretching out his hands cross-wise, with his right hand touched Ephrem standing on the left; and set him before Manasse standing on the right, whom he himself touched with the left hand.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)