In A Box of Matches, Nicholson Baker writes:

Last year, Claire gave me an ax for my birthday, and I began using it to chop up the scrap wood that the contractors piled up where they were reconstructing our slumped barn. If you bring the ax down really hard, right in the middle of a six-inch board, the board will break into two longways, and the grain of the breakage will sometimes detour nicely around a knothole. Then you chop across the grain.

Having recently been the recipient of a large load of contractor's scraps, I can confirm the part about breaking boards longways. If you hit a board just right, it takes only one blow for a lengthwise split. Plus, the resonant cracking sound you get for your effort is a satisfying reward.

However, using an ax to chop across the grain is a waste of sweat and energy. I tried. And tried. It results in many, many fruitless swings, so no person in their right mind would use this method to cut boards into fireplace-sized lengths. Better to pull out the big gun, a circular saw.