Shift the precise moment when the triangle turns green. If done correctly, you will get a perfect shift and a short boost. To time this easier, ignore the triangle and focus at the tachometer which displays your engine's RPM on the right. Wait momentarily when the needle gets to the green zone, then tap to shift gears.

By playing with the sound on, you will also have an additional indicator as to when to shift. This will also be extremely helpful in Blind Races.