If you want to use a 400 speed as if it were 1600, you set the ISO to 1600 and you have the pro lab push process 2 stops. A 400 as 100 you set at 100 and pull process 2 stops.
Kodak Portra 400 has such wide latitude that it is not likely you have to do either for such a narrow exposure range.
Larger version ->
http://www.fototime.com/B1379B2FE749C83/orig.jpg
For this example of Kodak Portra 160, I applied some scanning and post correction. It too has extreme latitude.
Larger version ->
http://www.fototime.com/3736A3E18E3D020/orig.jpg
Example of Kodak BW400CN.
Larger version ->
http://www.fototime.com/0A2BFD8BCB1F695/orig.jpg
I've conducted this same test on various films and generally speaking, most all negatives (C41) and b&w have extreme latitude. For those new (or renewed) into film, you might want to conduct your own test of at least + or - 2 stops just so you can be aware of your own system's tolerances and post processing requirements.
Knowing this, you can apply it in your own photography much like knowing post work with RAW files. For instance, I came upon this scene that my meter indicated needed 1/60 but I wanted 1/2 to slow the water down. Of course I didn't have - nor need, ND filters since I knew that Kodak Ektar's latitude will allow me to get good results so I shot the scene as I wanted.