I'm writing a scene where a character lifts another one up above their head and slammed them down to their knee?
Are you looking for short term consequences in the back breaker scene itself, or the long term consequences for the target? Or maybe both?
Who the hell is Bane, and how am I supposed to react if he broke my back? Pretty sure I'm not going to invite him over for Thanksgiving or anything.
Well, this would probably result in some form of spinal fracture, dislocated vertebrae, or perhaps even both depending on how exactly this unfolded. So the immediate impact is fairly obvious: immense pain. If there's nerve damage, you can get numbness as well as pain at the site of the injury. Reflexes and muscle strength can be impacted as well. Bladder and bowel problems are possible, as is paralysis if there's spinal damage. Longer term, the target is going to need surgery, a back brace, and physical therapy. You can read up on the specifics here: https://www.healthline.com/health/broken-back#symptoms Recovery can take months or years in the worst cases. Even then, the target might not be able to return to all their old activities. To take the titular example, Bane's backbreaker probably would've ended Bruce's career as Batman in a more grounded setting. And DC's writers seem to have understood this; that's why they had his back injury healed with metahuman powers in the second part of the whole Knightfall story arc.
I think that he would faint right after the blow. Then he would need surgery and long-term therapy. A friend of mine once fell of his motorcycle and broke a vertebra (spinal bone). It took him months to recover. Now, he is able to ride his bike again. He is very careful upon anything he does, although he seems pretty fit. Whenever the weather gets cold, his back hurts more.