Joe- Hey, next time you are here please come train with us! We just had Inagaki Sensei here at our dojo and this was one of the things he said to do. Those guys in Iwama seem to do alright with a couple hundred cuts on the tanren. I will keep an eye on any back pain though. Thanks!
Jason- That helped a lot! Now if I could only get it softer so I can practice in the dead of night when I feel like it...lol,
FWIW, you should be able to get the same benefit using a rubber mallet or even the palm of your hand. I can't imagine a rubber mallet hitting a stump or stone would produce quite as sharp a sound.