There is a technique for splitting wood with an axe, that involves, just at the point of contact with the end grain of the wood, a small twist is applied through the axe handle. The twist causes the severed grain of the wood to fly sideways, which is what this device does without depending on the judgment of the axe handler, as to the exact instant at which to make the small twist.
And there is that further complication, that some varieties of wood are just a lot harder to split than others. A willow or slippery elm has very tenacious cross-bonding between the wood fibers, and will shred much like a piece of rope with the standard techniques. But wait until the temperature is well below freezing, when the moist tendrils of that wood fiber are crystalized ice, and even these very moist woods will pop apart like pine.
An axe is actually a very simple machine, a wedge, and relies of the surfaces of the inclined plane to push the two parts of the wood grain apart. An adjunct to the axe is the splitting maul, or the splitting wedges, that are dropped into the partially opened split to further widen the gap formed by the first strike of the axe.
I know, as a kid my dad Ole had me out there in sub-zero weather learning all these tricks. If you wanted to get back in where it was warm, you learned quickly. You also worked up a sweat first, so the wood would warm you up twice.
Freezing water expands...so the ice will crack the wood.