As with machine shop operations, it is often necessary to hold objects whilst doing other operations. An example is when using a dial indicator.
A Noga Arm device is often employed for this, and it is often held on a base which has a magnetic switch built in.
There are other brands of these, and they can be found inexpensively at many online merchants.
Disclaimer: eMail comments to me at OTBookOfKnowledge @ Gmail.com. The process of woodturning involves the use of tools, machinery and materials which could cause injury or be a health hazard unless proper precautions are taken, including the wearing of appropriate protective equipment.