The intensity of the illumination falloffs very quickly from the external strobes and hotlights we use to light our models. Unlike portrait shoots, frequently shibari type shoots involve models moving around in different positions during the shoot. This can easily cause over and/or under exposure problems for different parts of the models body in the same image.
As an example, think of a suspended model slowly spinning in a horizontal pattern. If a 5 foot model’s head was 3 feet from the light source, the light on the model’s head would be 1/9 as strong as at the light source, while the light on the models feet extended straight out from the light source would be 1/64 as strong as at the light source in the same. Then as the model slowly rotated, the illumination pattern would change completely. With the same light setup, with the model rotated 180 degrees, the light on the model’s feet would be 1/9 as strong as at the light source, while the light at the model’s head would 1/64 as strong.
Here is a good video by Mark Wallace, demonstrating the effects of light falloff, and with some suggestions on how to use or compensate for it.