Pushing and pulling is the process of over-developing (pushing) or under-developing (pulling) as a creative technique that can go someway to counteract over or underexposure. It isn't, however, something that we would recommend being used as a 'save all' method of counteracting, or saving accidental over/underexposure. Pushing will typically increase contrast, can occasionally cause colour shifting and will still not be able to retrieve detail from shadows, which are ultimately non-exposed parts of the film. Whereas pulling can result in decreased contrast, and muddy/washed out highlights.