Read Exodus 17:11 Using Other Translations
And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed.
Whenever Moses held up his hand, Israel prevailed, and whenever he lowered his hand, Amalek prevailed.
As long as Moses held up the staff in his hand, the Israelites had the advantage. But whenever he dropped his hand, the Amalekites gained the advantage.