This is a great training tool. It will let you do both slapping and penetrating type conditioning exercises across pretty much most of the muscular/fleshy blocking surfaces, not just your arms. There is a really noticeable increase in your toughness and conditioning when using regularly and intellegently. The slapping/surface actions are also excellent for raising a flush on the skin's surface and bringing the blood up for a good, purifying sweat too. Just be careful you don't overdo it. Using different surfaces and choking-up your grip let you vary the intensity. I've also used the hole in the handle and suspended it from the ceiling for additional impact training.
Very Useful 
