Have you ever encountered the situation where the trainer resistance lags behind the actual terrain? If I understand this correctly, the reason is the trainer is slow to respond to commands sent from the local PC.
However, Zwift knows well in advance the terrain and slope (given riders speed, and no breaks). Hence, it can (most times) send a resistance “script” forecasting, say, 5 seconds of resistance into the future to the trainer, which would act upon it as per time indicated (say “change gradient to 10% in 2 seconds”)
Granted, this requires trainer support (its kind of a chicken and an egg problem - no use having this feature unless there is trainer support, and no use having trainer support unless some app takes advantage of it…) However, I would assume this stands to make an impact wrt the game being more “realistic”, thus an interesting area to explore.