is a colloquialism for the CoP, or Center of Percussion. It is a physical property of any swung object, and can be calculated by feel.
The Center of percssion is opposite from the center of balance or point of balance in a sword and most knives. In an axe, the point of balance and the center of percussion are very near each other.
Sir Richard F. Burton writes in his 1884 masterwork _The Book of the Sword_: "By the experiment of cutting along the entire length of the blade, and then comparing the effect, the swordsman at last comes to a point ... where there is no jarring feel, .. and the whole force of the blow becomes effective." This is the center of percussion.
He goes to great length to describe a buddy's scientific means for determining the CoP, but it is impractical for me to count vibrations at points of a sword using pendulums and such.
The CoP in all HI khuks is the hardesst part of the steel, for good reason.
Keith