Mark Kalkwarf discusses Pushing FIlm

Shooting in low light situations with no flash can be a tricky thing, you end up shooting at shutter speeds that are too slow and the result is a blurry image. By pushing your film ISO 1 or 2 stops you can overcome this problem, here is how.
Film sensitivity to light is rated using the ISO system. ISO 100 film is less sensitive to light than ISO 800 film (The more sensitive the film is the less time it needs to be exposed for). If you set your camera to ISO 200 and you are actually using ISO 100 film you are pushing the film one stop. By pushing the film you are able to gain 1 or even 2 stops, which basically means a faster shutter speed.
Let’s say you are shooting with ISO 100 film and your 24mm f-2.8 lens wide-open gives a 1/60 sec shutter speed (the amount of time the shutter will stay open for). This might not be a fast enough shutter speed for you to capture movement. If you change your ISO setting on your camera to ISO 200 (so the camera thinks you are using ISO 200 film), when you take another reading you would have gained a stop so the shutter speed will now be 1/125 sec.
By pushing the film you are actually underexposing each image but when you have the film developed just tell the lab by how many stops you have pushed the film and they will correct the exposure during processing.
The down side to pushing film is an increase in contrast and noise in your images but if you are a fan of both of these things, (as I am) there is no problem.