Is it an expression for pre-transition women?

"Boy mode" is the term for when one is actively presenting as male, either because one is still early on and HRT hasn't had effects that can't be hidden yet, or because one is socially obligated to present that way for some reason. Like even when I'd been on HRT for over a year I'd still go to the liquor store in boymode because I didn't want to have any issues with the gender marker on my ID, and I did that until I'd gotten that updated. I'll also add that "boy mode fail" is sometimes considered a milestone, which is getting correctly gendered by random strangers despite one's presentation.

More generally, "boy mode" or "girl mode" is just the term for presenting as a particular gender and can be used by both trans masc and trans fem people for how they're presenting at any given time.