Tameshiwari means "trial by wood". It is the breaking of wood boards with the body weapons.
Traditionally this is done on a pine board as it has the same density as human bone.
It is said that to break 3 boards with a technique means you have mastered that technique.
Kyukushinkai have taken breaking to it's extreme, breaking bricks, stone, ice, and using all body weapons including the head.
TKD break boards with a range of kicks.
For 1st dan at ST Karate you need to break a board with both fudoken clenched fist and shuto knife hand.
For 2nd dan you need to break two boards.
For 3rd dan you need to break a stack of 3 boards with each. This shows you have mastered both the closed fist and knife had of karate.
