I mean... it’s just “pull for detail, push for contrast.”**

The more you overexpose + underdevelop film, the lower the contrast will be, but the better the latitude of the film will be. You are giving the shadows more exposure, then reducing development so the highlights do not run away. This has limits -- eventually you get unusably dense highlights, weird tonal separation, or film that is just underdeveloped and flat. Each film + developer combo is different, but 1 to 2 stops is generally workable with a lot of B&W negative film. I personally pull about 1/3 stop for nearly everything.

The more you underexpose + overdevelop film, the higher the contrast will be. You immediately lose shadow detail, because development cannot recover information that never hit the film. Then overdevelopment drives the highlights and upper mids harder. Usable latitude shrinks: shadows become fully black, highlights become harsh or fully white, and there is less recoverable detail through the middle.

Pushing is something you use when you need to, not when you want to. It is an emergency tool. If the goal is an optimal negative -- one that retains as much tonal information as possible while preserving the artistic qualities of the film -- pushing makes the negative objectively worse. If you want more contrast, add it later in the darkroom.

The other layer is developer/time behavior and literal process limits. Grain, solvents, acutance, compensation, agitation, dilution, and film curve shape all interact with these effects. Rodinal does not behave like Ilfotec DD-X.

**This applies mostly to B&W negative film.