The user kneads their chakra into flames, which are then manipulated into a genuine-looking dragon. The flames are so hot that it could turn its victim into ash in a matter of seconds. Since the art of having the flames obey the user's will is highly difficult, only a few shinobi have mastered it.
In the anime, when used by Hiruzen, this technique is depicted as a high-pressure stream of fire, which has no definitive shape. It would also seem that, by increasing the output of chakra to the flames, they become hotter and appear whitish in colour.