Heavy tapered steel rod for making rings round (usually because the customer has 'squished' them!)
A very thin ring can be forced down the mandrel with your fingers, it's no more complicated than that.
Thicker rings must be pushed firmly down the mandrel with your fingers, but then tap the shank with a nylon hammer as you turn the mandrel, and gradually 'tap' more and more firmly until it becomes round.
Many people use a ring stick instead. That's fine with me, they can buy a new ring stick every few weeks when they've completely ruined it. Better to use a mandrel.