During a battle, Poe is forced to crash land and is knocked unconscious. Either Phasma or a OC stormtrooper manages to capture him and try to use his body as a way to try to force Finn to surrender. Finn initially doesn't and that causes Phasma/stormtrooper to snap Poe's neck (not killing him but Finn doesn't know that). Enraged at thinking he just saw Poe get killed before his very eyes, the force suddenly "awakens" in Finn and he uses it to kill that person.
He would then realize that Poe is still alive, albeit clinging to life, and then use the force to help heal him.