What's not being mentioned here about pushing film is that when shooting in flat low light such as dark cloudy days most of the tones are close together, and the subject brightness range is rather low (excluding sky). By pushing film up to one stop we are really placing the overall scene up to one zone lower, so the shadows are placed where they should be anyway. Then we extend development to bring the midtones and upper midtones back up. I regularly shoot Tri-x and HP5 pushed up to one stop in flat lighting (overcast) and overdevelop to gain very nice contrast. If spot metering is used that's a whole different story.