Why would one push or pull a film? I can see three reasons: 1) To compensate for certain extreme light conditions (snow, night, etc). 2) When pushing, one can use the film in less light circumstances. So the need for a flash is circumvented. Drawback: the film generally has a worse performance. 3) If one wants special characteristics, grain, etc. But what are these special characteristics? Any info on point 3)?