Ollivander’s eyes cold eyes fixed on Draco as he proceeded to explain, "Procuring dragon heartstring involves either slaying the dragon and taking it’s heart without it’s consent or in rare cases the dragon offers it’s heart. When the dragon offers their heart, a part of their soul gets transferred into the heartstring. The only people who can conjure the Effluvium, or that wisp of smoke, are the ones whose souls call out to the soul of the wand. It seems yours did. And the dragon answered.’