Copied from: https://www.quora.com/Carpentry-What-is-wood-checking-and-how-can-it-be-prevented
Checking is caused by improper drying of wood, especially in a kiln. There is a good explanation of checking and case hardening causes and remedies available from the Forestry service. See
In my experience you will see very little checking in properly air dried wood (in dry shade, with sticking for airflow and weighting to prevent warping. The rule-of-thumb being one year per inch of cut lumber thickness.
Kiln drying speeds this process but always changes the wood. The outside of the boards become more brittle (case hardened) and if this brittle layer becomes too thick you will get surface checking (cracks), especially in plain sawn wood. The ends of the boards also tend to dry more quickly building internal stresses in the board and causing splits at the ends of the board. In the kiln drying process the ends are often painted or waxed to lessen the problem.
Logs left to dry uncut will often develop large surface cracks. This is because the wood is drying on the outside but not on the inside of the tree. The outer layers shrink as they dry and eventually split as the pressure of shrinking against a static core causes the wood strands to separate. This is essentially the same process with all wood checking... If the outside (or ends) dry and shrink much faster than the inner core the pressure buildup will cause checking (a split).
If wood is milled into smaller dimensional lumber (no bigger than 2x6) before it is kiln-dried, checking is minimized. A larger piece of wood will almost inevitably check as it dries out. You can minimize this tendency in milling by either rift or quarter sawing. When wood is milled this way, it tends to dry in a very stable way and is less likely to check, warp or split.