Push development extends the development time for film exposed above its box speed - for example ISO 400 film shot at 1600 in low light is "pushed 2 stops." Expect stronger contrast and more pronounced grain; many photographers love the look.
Pull development is the opposite: shortened development for film that was overexposed (shot below box speed).
When ordering, tell us the difference in stops between the ISO you actually set and the film's box speed. Push/pull applies to the whole roll, so you cannot change ISO mid-roll. We support ±1-5 stops.