I read this short story decades ago in a German translation in some anthology, but I believe the original was in English. Or was it in German? I'm not even sure. I'd like to find out the name of the story and who wrote it. (And the name of the fictitious device that plays an important role in it.) It's very likely that the story is from the 80s or earlier.
A human fighter pilot in an interstellar war is downed on a planet where some alien race lives that works for the enemy, but has had almost no contact with humans before. He is imprisoned and comes up with a plan. Out of old wire and wooden planks he starts to form little devices. Then he speaks through the wire, as if somebody else was there. The guards question him and he tells them that humans use these devices to communicate with their invisible spirit companions. He tells them that his companion is angry, and that he cannot control him, although he tries to. Something bad happens to one of the guards, and they get really anxious about their human prisoner. Anything bad that happens is swiftly attributed to the invisible spirit companion.
The pilot continues with his plan despite setbacks and complicated questioning. After a long back and forth, he finds that his plan has done more than grant him his freedom: the alien alliance has decided to end the war because invisible human spirit companions are too dangerous.