[sarks_nz]

I did Kyokushin karate to Shodan. The board we broke were solid pine boards. They are definitely not "cut". My technique was a little off, and I wrecked my knuckles for months. They ended up permanently moved.

So maybe all karate isn't equal.

[peddling-brink]

Not cut as in pre-broken (though I’ve seen that too), but cut as in the original action which turned the log into a board. The boards are formed the “wrong” way if you wanted strength, like for building.

Take one of your boards and hold it with the grain horizontally instead of vertically. Try not to break your hand.

[harimau777]

I think that most people just take a regular board and cut it into several lengths. Since the grain generally runs along the longer dimension of the board, that means that you break the board down the middle.