It's a game mode where the game runs at 60fps instead of 30.
Because of this, you're limited to three tracks, each one having less scenery to handle the 60fps.
It's definitely a cool mode.
Cool thing about GT HI-FI mode is that, with track loader gamegenie code, you can run this mode on any track , and all works well with 60 fps .
So basically the three night tracks are not the only ones you can drive on
Need to make a video on it when I come home.