He uses the truck to determine the suitability of his victims, bumping into them from behind to frighten them, this is so that he can smell their fear, and pick out what part of the human he wants.
He uses the truck to determine the suitability of his victims, this is. The suitability of his victims is bumping into them from behind. This is to frighten them. He can smell their fear. He can pick out. He wants.