Please explain ring of fire ?? 39w+3D ?!

Samantha • Mommy of a princess 03.25.18 💕

I keep hearing ring of fire ? Can someone explain this to me ? I’m getting the idea it happens during vaginal birth , sometimes can be felt through and epidural . What I’m having a hard time figuring out is what causes it ? Babies head ? Babies shoulders ? Tearing ? Etc .

Thank you ladies !!