Portal Culling

I'm trying to use Portal culling in my 3D scene, but I don't understand very well How I have to use it. I've seen that Unity3d have an automatic portal culling, but I don't know how build mesh have to be to permit me to use automatic portal culling. I've tried to enabling portal culling, but I've seen in the Occlusion View (selecting Portal) that a transparent mesh appear abread my mesh. But these are very different. How have to be the mesh for use correctly Automatic portal culling? Is there a way to create manually portal graph? Am I doing something wrong?

Thank you.