Pushing and pulling film has several different meanings and uses. Pushing usually means underexposing and over developing to shoot is dim light or to increase the shutter speed in good light to stop the action while keeping the aperture wide open to blur the background. If you have tested for personal IE or ISO and find that shooting HP5 gives you details and the shadows at box speed and you decide to push to 800 then you need to make adjustment to the development time. If on the other hand if you find that 800 or 200 is the speed that gives you shadow detail without blocked highlights then that is not a push or a pull, it is your normal speed for your camera and developer combo. So if personal ISO is 200 and you shoot at 400 that is a push. If you your personal ISO is 800 and you shoot at 800 not a push. If your personal ISO is 800 and you shoot at 400 that is a pull. When pushing film the old saying was to expose for the highlights and let the shadows fall where they may. The more you push a film more you compress the shadows. Pulling was done for a number of reasons. One is that you are shooting a very low contrast scene and you want to increase the contrast, you over expose the film then under develop so as not block any highlights, if any. If you only want to increase contrast by 1 zone you may need to just add development time. The other reason I can think is that back in the day you might be using a high speed film like Trix but have camera with slow shutter speeds and were shooting in very bright light so you pull the ISO and underdeveloped. In the 60s I shot a lot of TriX and GAF 500, my first 35mm was a Kodak Retina C, top shutter speed 1/500th, there were times I wanted to shoot wide open but the scene was outdoor mid day, I had to pull the ISO to 200 and decrease the development time to compensate. Once I got a Spotmatic with a top shutter speed of 1/1000 I did not need to pull. Ansell Adams used the terms expansion and contraction when discussing pushing and pulling film to control contrast.