Here's an excerpt:
"If you go into your character’s life and ask yourself what traumatic incident might have occurred in your character’s life when he or she was between the ages of 8 and 18, you can create an emotionally charged incident or event that strongly influences and impacts your character’s life. Do this exercise and you can discover what the Circle of Being might be: the death of a parent at an early age; the family’s move to a new city or country; the betrayal of a friend; an incident or event that causes a severe traumatic scar like Mystic River. It could be a physical event or injury. In Seabiscuit, the four main principles are strongly affected by loss: Charles Howard (Jeff Bridges) loses his son; Red Pollard (Tobey Macquire) loses his parents; Tom Smith (Chris Cooper) loses his freedom; and Seabiscuit is deemed worthless and given away. The horse is sold to a trainer who forces the animal to train according to the way he thought he should be trained. The animal literally loses his identity; he doesn’t know “how to be a horse, anymore,” as Tom Smith says."
Circle of Being just seems like a fancy way to say "Give your character a backstory that will resonate in their present."