As in its exact functionality.
When DF first started talking about PS5's boost mode, some users here thought it meant an automatic 4x resolution multiplier for PS4 games and whatnot.
Well, the PS4 Pro boost mode and PS5 boost mode were the same. So the PS5 Pro mode was assumed to be the same:
- Games with unlocked framerate get a higher framerate
- Games with capped framerate get a more steady framerate
- Games with dynamic resolution get a higher one
- I have no idea if loading times were improved in the boost modes, but I'd bet they were and also should happen the same now too
Boost mode means unlocking the extra horsepower while in BC mode, which in practice means it has these effects not needing any action from the devs.
Regarding how exactly it does impact each game, obviously will depend on each game, as happened before.