During developing, add more time to the development, e.g. 10min for
Tri-X400 pushed to 1600 for Tmax dev.
You are talking here about
pushing on film development.
Definition of pushing on Wikipedia:
Push processing in photography, sometimes called uprating, refers to a
film developing technique that increases the effective sensitivity of
the film being processed.2 Push processing involves developing the
film for more time, possibly in combination with a higher temperature,
than the manufacturer's recommendations. This technique results in
effective overdevelopment of the film, compensating for underexposure
in the camera.
Source: Guide to film photography
When pushing or pulling film at different EI speeds, you must over- or
under-develop the film to compensate for over or underexposing the
film. Generally, pushing the film to another ISO makes the development
process much easier. If you push a 400 speed film to EI 800, you
should develop the film based on the development time for ISO 800
film. This will give you a solid starting point as you learn to push
or pull process film.
This technique applies to both colour and black&white films.
Talking B&W now: As you (probably) already know, the longer you keep the film in the developer, the more the silver halides on the films are reduced to metallic silver which will then get fixed on the film during the fixing process.
That means, the more you keep the film in the developer, the more "light" will be caught on film, the more the exposure will be pulled during the development.
You should nevertheless always read the specifications of the developer.
Quoting again from
Guide to film photography:
Chemical developers have also provided some general rules for push and
pull film processing. Kodak recommends that when push processing, you
should increase the development time by two minutes for each camera
stop of underexposure. With pull processing, the development time
should be decreased one minute for each stop of overexposure. Ilford
recommends increasing development time by 20% to boost contrast for
Both companies provide basic data sheets for push/pull processing
times similar to their sheets for standard processing times. Please
consult the appropriate company for more information.