From the Harry Potter Wiki
Arania Exumai is the incantation of a spell used to blast back acromantulas or other large spiders. If the spell is used on anything but a spider it will produce a large black scorch mark, and thus it is not recommended for use on humans.
Harry pretty much blasted Ron in the face with the spell in the car in the Chamber of Secrets, how was Ron not harmed?
The spell (used in the movie) was used in the presence of spiders. It's really that simple, I think. I used a Search function on my PDF of the book and it produced "No results." It seems to be an invention of the film and one only used in the presence of spiders.
Why would Harry use it when there were no spiders around?
It didn't harm Ron because it did not hit him.
Ron was being menaced by a BIG acromantula. Ron saw Harry pull out his wand to help him. Ron helped Harry by leaning back on his seat. If you look carefully at the scene, you will clearly see him do this. Now Harry had a clear shot at close range. Young as he was, it was hard not to miss -- and he didn't.
I think if it can only be used on arachnids, why would it harm a human if it was aimed at a arachnid? If he said it with no arachnids around and just Ron, I expect it would leave Ron with the black scorch mark. This time it did not because there was an arachnid around.
The spell cannot harm humans but it can leave black scorch marks on humans or other things that the spell was used on. Spiders are the only thing the spell actually works on
This specific spell was only used in the second movie of Harry Potter and was never actually in the books, it was meant to blast back spiders acromantulas specifically.
