effects of pulling b&w film ?

[sequitur]

went to kodak webbie where the introduced tmax and said the photog pulled tmax100 to ei50 that affected something about the tonality.

so what are the effects of pulling a b&w film, or the reasons that one would pull a b&w film other than because of the grain ? how does pulling a film affect the tonality of the negative output ?

any help/enlightenment is appreciated.. thanks !