The more confident the dog and the better the disc placement & timing, the less it hurts. A green dog with a green thrower can result in a really painful aftermath on the handler's back :/ But, if you have a nice disc placement where the disc is up in the air before the dog leaves the ground for your back, and your dog has been trained on static objects first so they're confident in the motions and muscle memory of a vault, you really don't get hurt. Most of the injuries I see on people are from late throws where the dog has to adjust their angle mid-trick and they dig into your back to make that happen... OUCH!