Want to get a good bop on the back of the head; this is an excellent way to do just that. A flail is a fiendish device that used to be used for thrashing grain. I understand it is still used in some third world countries. It has also been used as a deadly weapon. In that case it is called a military flail, and the swingle, the business end, comes equipped with a nasty set of metal spikes.
A flail is a simple device made up from two pieces of wood a handle or staff about five feet long and a swingle that is about two feet long. These two pieces are joined loosely together by a rawhide thong. As simple as it looks it is an ancient tool used for thousands of years to pound the grains from the heads of a sheaf of grain. They were commonly used until the invention of a mechanical threshing machine in the 19th Century. The job of flailing grain was very labor intensive just like all the other ways of doing jobs that today we take for granted.
In use the handle of the flail is grasped by two hands. The left hand cups the butt of the handle, and the right hand grasps the handle somewhere where it is comfortable. The grain is spread out on the threshing floor or other hard surface used as a threshing floor. A barn floor was usually used for this purpose. The flail was used in such a way that the swingle came crashing down onto the sheaves of grain so that the blows knocked loose the individual pieces of grain along with the chaff. After many strokes with the flail the user had knocked all the grains loose from the straw. With a three tined pitchfork the straw was removed, and the grain along with the chaff was swept up, and placed in a basket for another process called "winnowing." Afterword's more sheaves of grain were spread out on the threshing floor to be in its turn threshed with a flail. Usually the whole family worked together doing this job, so that you had to mind your own swingle, and watch out someone else in your family didn't brain you with a clop along side the head. The littlest children in the family were tasked with the job of removing the straw, and sweeping up the grains and chaff.
It was necessary to partially turn the handle with each stroke of the flail otherwise the swingle was just as apt to brain you as pound the grains loose. This was a process that you had to learn by practice, and in the process you bopped yourself in the back of the head many times until you got the rhythm of the job down pat. It's no wonder that the old timers were a bit punchy with a job like this one.