4 general hitboxes. Each one comes out in a different time during the attack. The first ~fourth of the attack (timewise) hits them upwards, Second ~fourth hits them down and behind (commonly called reverse tailspike). Third ~fourth hits them down and forward (standard tailspike). Last ~fourth hits them slightly up and behind. So it's not so much where you hit them as when you hit them during the attack. The hitbox extends his entire tail-length, with more severe knockback occuring seemingly near the outer edge of his tail.
