Before reading this article, read these ones:
The basic area to flog is the upper back on each side of the spine. Try to stay off the spine to prevent injury to the vertebrae and related tissues, and the skinnier the person is the more important this is. Also avoid swinging the tails down the back onto the hips or upper butt, or else this may cause a different kind of pain that is undesirable. Rather than a full swinging motion (this isn’t golf :P ) that carries the tails down the back, stop the flogger immediately after it hits or even pull it backwards in a snapping motion right before it hits. By varying this motion you can control how much of the tails connect with the back; more will cause greater thud while hitting with just the tips will be stingier.
Read more following these articles:
Wrapping
Careful while flogging
