Complex sentence: A film with a particular ISO rating can be push-processed, or "pushed", to behave like a film with a higher ISO, by developing for a longer amount of time or at a higher temperature than usual.
Split into simpler sentences: A film with a particular ISO rating can be pushed to behave like a film with a higher ISO. In order to do this, the film must be developed for a longer amount of time or at a higher temperature than usual.