Hi Farid, the takeoff shift is derived from the takeoff performance calculation when using a shifted departure point. ie. Intersection takeoff.
Its purpose is to provide a final FM position update at takeoff thrust when GPS PRIMARY is not available. It also gives you more accurate climb predictions based on you new departure point, which are only for advisory.
It is commonly used at long runways were it can be advantagous in saving time and fuel, and even sugested by atc for traffic sequencing.