‘And when he came to Lehi, the Philistines shouted as they met him.'
On seeing this ferocious man, who had killed so many of them, bound and helpless, the Philistines let out a shout of triumph and gloating. Now they could exact their revenge. He was theirs for the taking.
‘And the Spirit of Yahweh came mightily on him, and the ropes that were on his arms became as flax that was burnt with fire, and his bands melted from his hands. And he found a new jawbone of an ass and put out his hand and took it, and smote a whole military unit of men with it.'
Once again Yahweh acted through him, and he burst the ropes that held him which seemed to melt away in front of them. Then he seized a new jawbone of an ass, its newness ensuring that it was solid and effective, not brittle, and used it as an effective weapon. With it he effectively destroyed a whole large military unit, presumably the one that had come to receive him from the hands of the men of Judah. They were, of course, taken by surprise and probably panicked for he had a fearsome reputation.
So were Samson's effective actions against the Philistines increasing in magnitude. First thirty men (Judges 14:19), then ‘a great slaughter', possibly nearly a hundred (Judges 15:8), and now ‘a large military unit', well over a hundred.
But the seizing of the jawbone was a careless act, for as a Nazirite he was under a vow not to come into contact with dead things. Perhaps this was a sign that he was becoming careless with regard to his vow. He was beginning to feel that he was above restriction (contrast Judges 15:8 where he used his wrestling ability and his bare hands).