I read a story in a sci-fi magazine anthology in the early '80s about a "primitive" culture on some faraway planet. The story started with a young boy practicing fighting in a group of other young boys, each armed with a wooden stick and shield, wearing loincloths and painting the picture of your standard low-tech backwater primitive culture.
Later, the planet is invaded by what we would think of as standard Terran Marines with armor and laser guns, and that same boy is now grown and fighting back in guerrilla warfare in the jungle with the rest of his people. Now they use high-tech shields that absorb the laser fire and use the energy to power their lances, which are a sort of combination of lightsabers and point-to-shoot wands.
I don't remember getting the impression that they were given any technology, I think the overall moral of the story was that they were never really primitives at all. They could have used this technology at any time, but chose to live as simply as the demands of their surroundings necessitated, and only began implementing it to counter the invaders.
There was some illustration with the story that showed the shields being transparent and a cable leading from the shields and wands to a backpack power storage.