Charles Ellicott Commentary Exodus 17:11

"And it came to pass, when Moses held up his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed." — Exodus 17:11 (ASV)

When Moses held up his hand ... Israel prevailed, etc. — In order to teach the lesson of the value of intercessory prayer, God made the fortunes of the fight to vary according to whether Moses "held up his hand," or allowed it to sink down. It is not probable that the Israelites were directly affected by the bodily movements of Moses, or indeed could discern them, but Moses, Aaron, and Hur were struck by the fact that the fluctuations in the battle coincided with the motions of Moses’ hands.