meaning - What's a yellow line running down somebody's back

The only line between good and evil is the yellow one running down your back. What line does it speak of? I'm also not sure what meaning does "run down" have in this context. The source of

Panthers place McCaffrey on IR; RB can't return until Week 9 - The, back in game meaning

MDF Personagem P Pintinho Amarelinho Galinha Pintadinha - Dinove Arte e Festa, galinha desenho filme

Back in the Game (TV Series 2013–2014) - IMDb, back in game meaning

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano

Meaning of Back in the Game by Battle Tapes, back in game meaning

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano

From Mechanics to Meaning and Back Again: Exploring Techniques for, back in game meaning

From Mechanics to Meaning and Back Again: Exploring Techniques for, back in game meaning

I've been meaning to get back into the game after getting bored of, back in game meaning

Meaning of Back in the Game by Battle Tapes, back in game meaning

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano