Warren in Lancaster, PA
The wood shrinks as it dries. If the wood dries rapidly you will have it relatively dry on the outside while still quite wet on the inside. The inside is still swollen but the outside shrinks and is stretched and cracks. So you want it to dry slowly so there is not such a great difference between inside and out. Keeping the timber out of the sun and the wind and out of heated spaces is helpful for slow and steady drying.
The moisture generally exits the end grain faster than the side grain. This presents another special problem. The cross section at the very end shrinks and is much smaller than just a bit away from the end so end checking occurs. Usually we just discard the very end of the timber, but for valuable stuff we sometimes coat the end to keep it drying evenly with the rest of the piece.
Rough turning green removes the outside and makes the piece slightly smaller in cross section so drying can go slightly faster.