Charles Ellicott Commentary Psalms 78:57

"But turned back, and dealt treacherously like their fathers: They were turned aside like a deceitful bow." — Psalms 78:57 (ASV)

Turned aside ... —Better, turned like a relaxed bow. (See Note to Psalm 78:9.) The bows of the Hebrews, like those of other ancient nations, were probably, when unstrung, bent in the opposite direction from the shape they took when strung, which makes the figure more expressive of a disposition that cannot be relied upon in a moment of need.