Then he suddenly gave the signal and launched his attack.
Paraphrase: Jugurtha attacked Metellus. This was a trap that had been laid down by Jugurtha because he knew he wouldn't win fairly. He let Metellus walk around the village and caught him off guard when he least expected it. Metellus and his army were now trapped. This once again shows change over time of how war has advanced from past wars and different techniques being used.