Most of my bowls are 'twice turned'. I use the lathe to rough them out from green, unseasoned wood and let them dry before turning them again. They warp in the drying, but their shape doesn't change after the turning is finished. The walls are left thick enough for the distortions in the bowl to be removed and the shape refined during the second turning. But some I turn and finish while they are still green, without waiting for them to dry.
Green wood comes from a freshly fallen log, full of sap. A block of green timber, the fresher the better, is put on the lathe and turned to shape. The unseasoned wood is softer and turns more easily than dry wood, but it's a messy business. The sap is thrown from the wood as it spins, and can literally drip from the ceiling. The tools have to be protected to prevent rust and I put up a shower curtain to limit the mess.
I turn the bowl to its finished shape, thin-walled so it can dry quickly. The thin wood is able to move, relieving the stresses that develop. I have to work fast, without interruption - if the bowl changes its shape before the turning is finished it may be impossible to complete. Sanding green wood is difficult, because the abrasive clogs. I prevent this by spraying the wood with water to turn the sanding dust into 'mud' that can be washed away. When the process is complete, I put the bowl aside to dry. Over a period of days, the bowl warps, becoming oval or irregular in shape, often with a wavy rim. Areas of burr and distorted grain that were smooth develop a leathery texture. Sometimes cracks appear. When the bowl is fully dry it stops moving. I true up the base so the bowl will stand without rocking, then apply finish.
The completed bowl has a very 'organic' shape and sometimes an ancient, distressed appearance that appeals to many. Others prefer the more formal shape of a twice-turned bowl. I like both.