Albert Barnes Commentary Judges 15:14

"When he came unto Lehi, the Philistines shouted as they met him: and the Spirit of Jehovah came mightily upon him, and the ropes that were upon his arms became as flax that was burnt with fire, and his bands dropped from off his hands." — Judges 15:14 (ASV)

The cords ... became as flax ... - that is, were as weak against his strength as half-burned flax which yields to the least pressure.