Behind the flag lies a mask—yet another layer of secrecy. You’re standing in front of a crowd, giving a rousing speech. Hiding your face is the flag of your country, and behind that flag you’re also wearing a mask, just in case the flag falls down. The extra layer of secrecy is just to be safe.
Someone jumps up on stage and starts peeling off your layers! You stand there, first goes the flag, then the mask, then ... your soul (そう) is laid bare. No, literally. You don't have a body, your soul was under that mask. And now it's floating there for all to see.