I’ve been writing about milling lumber by hand recently, and one thing that can happen when working with rough lumber, whether milling by hand or machine, is that the wood can sometimes move or warp after it has been planed flat. Sometimes this is due to additional moisture equilibration that can happen after the wood has been planed, and sometimes it’s due to tension in the board being released from planing.
Typically, if the movement is the result of internal tension, the movement will happen almost immediately, while movement due to moisture tends to occur over a period of hours. One way to mitigate some of the movement that can occur after planing is to not plane the wood to final dimensions right away. This is often referred to as skip planing, and it’s a technique can aid in keeping wood flat whether working by hand or machine.