Actually, it's rather easy.
Make a cut halfway through the thickness of a piece of wood at about it's midpoint - like you were working on cutting it into two equal lengths.
Grasp the piece of wood with the cut facing down.
Strike the end of the piece of wood against something solid - hard ground, a rock, another piece of wood.
The wood tends to split
lengthways at about the level of the bottom of the cut. The cut has acted as a stress-riser.
I have tried this method with red oak, Scots pine, white ash, cottonwood, silver maple, sugar maple, some
wood, and elm.
It didn't work with the elm, but have you ever tried splitting elm with an axe? (Batoning works better.)
It works better with pieces that are relatively long compared to their diameter.
Try it. Amaze your friends.