We talk often enough about requiring someone to "bend over" for a punishment but I must admit I also like to see someone "bending in". It's a reaction not seen nearly often enough in my view although these examples show that it is not entirely neglected by some of the main producers of CP material.
It seems an entirely natural reaction to having just received a tremendously painful whack on the butt to "bend in" as well as clutching at the affected area both to try to rub away some of the sting and, I assume, to psychologically move the target as far as is reasonably possible from the next blow. It has a futility about it as the movement is not going to really prevent further punishment - in fact if the punishment was continued on the back of the recipient's hands it would undoubtedly hurt even more and with the hands safely taken away, presumably being "bent in" like this would make it more difficult to move as the next blow landed and so could again make it hurt more. Nonetheless it does appear to be a natural defence reaction and in each of these cases, although photographed for professional CP publishers, the actual movement looks to be a fairly genuine pose.