I agree with all of the above. Spanking the back, calves, feet, etc can have lasting damage and it can be the kind of damage that doesn't show up right away. As time passes it can cause both nerve and muscle damage and can effect the rest of your life. There is also the immediate damage, stated above, bruising the kidneys or bruising/breaking the tailbone and spine. Those areas are not made for spanking and should be avoided.
As someone going through some major back pain right now, I have spent time talking to a neurosurgeon and Physical therapist lately. I have been cautioned to be very careful with walking and hiking. Their fear is my slipping and falling on my back or side and causing more damage to my spine and nerves. I turned that conversation to impact with objects and was told the same thing. An impact on the lower back, spine, tailbone or the sides of the bottom/hips can cause permanent damage. As Michglal said, it can cause sciatica and nerve damage....but it can also cause a ruptured or herniated disc...it can cause swelling and discomfort.....it can lead to permanent problems. It's not worth the risk.
We talk about the safety zone being from the middle of the bottom down to the upper thighs. There is a reason for that!!
Please be safe.