How is it possible for a ring singularity?

[Irfan Nafi]

If a star with enough mass to become a black hole is rotating before it becomes a black hole, then the singularity of the black hole will actually be a ring singularity. How is it possible?

 

[Borg]

If a star with enough mass to become a black hole is rotating before it becomes a black hole, then the singularity of the black hole will actually be a ring singularity. How is it possible?

Have you looked at the explanation in the Wikipedia article?