When the individual boards of wood are arranged so that the grain of the wood runs vertically (up and down), this puts one end of each board up so that the cutting surface is actually the end of many individual pieces of hardwood. With the grain aligned in this manner (up and down), when the knife strikes the surface during cutting, the grain of the wood actually separates and then closes when the knife is removed. This accounts for the "self-healing" aspect of the end-grain surface. The wood itself is not cut, but instead you are cutting between the fibers.