When a pot is at the leather-hard stage, it is turned on the wheel so that layers of uneven clay can be trimmed off with turning tools. I manage to get a nice set from eBay which I’m actually quite happy with.
Bowls can be held in position for turning by placing them on a this, firm layer of clay, previously thrown and then turned on the wheel head. Other shapes, particularly narrow-necked forms and enclosed shapes, can be turned in a clay chuck ( a thick, open-thrown shape of clay with a wide base which can be secured to the wheel with a coil of clay). The shape of the chuck will depend on the forms to be turned, but they are usually variations on a cylinder. If it is kept well wrapped in plastic and stored in a cool, damp place, a chuck can be kept for months.
Other turning technique is to wet the base of the pot and centre it on the wheeled. it will soon stick firmly and when turned it can be removed with a swift firm movement.