There are two basic wood grain alignments of a board used for board breaks.
Figure 1 is the top edge view a board cut from the center of a tree, where the grains are such that one face is the mirror image of the other. The amount of force needed to break the board will be the same no matter which side is struck.
Figure 2 is the top edge view a board cut slightly off-center from the center of a tree. The left side was closer to the center of the tree than the right side. Since the grain alignment is not symmetrical, on which side should the board be struck?
Striking the left face of the board as it is shown Figure 2 will cause the board to break more easily than striking the right face of the board. When a board with a grain alignment similar to that shown in Figure 2 is held on the edges by a board holder and the board is struck, three forces act on the board: two forces from the board holder pushing back against the board edges and one from the board breaker who is pushing against the board center. When struck, the board bends and the backside begins to stretch and split while the front side is compressed. The grain on the left side of the board is closer together and is more easily compressed. The grain on right side is straighter than the grain on the front side and thus easier to stretch. If the board were struck from the right side, the opposite would be true making the board harder to break.