When Moses held up his hand,… Israel prevailed. The elevation of Moses' hand, with the rod held in it, was an appeal to God for aid, and must be supposed to have been accompanied by fervent prayer to God, that he would help his people and give them victory over their enemies.
So long as the hand was upraised, the Israelites prevailed; not because they saw it, and took it as directing them to continue the fight (Kalisch), but because God gave them strength, and vigour and courage, while Moses interceded, and left them to themselves when the intercession ceased, It may be said, that Moses might have continued to pray, though his hands were weary; but only those who have tried, know how difficult a thing it is to pray with any intensity for a continuance.
Probably Moses' spiritual and physical powers collapsed together; and when he dropped his hand through physical fatigue, he rested also from his mental effort. To impress upon Israel the importance of intercessory prayer, God made success and failure alternate with its continuance and discontinuance, thus teaching his people a lesson of inestimable value.