When you do a flying overtake it means that you approach the vehicle in front of you at a high speed, then you quickly change to the left lane at a proper distance behind that vehicle in order to then drive past it. Due to accelerating to a higher speed it means that you need a shorter distance to overtake the vehicle. Besides, as it doesn’t involve braking nor explosive acceleration, because you gradually accelerate to a higher speed, it’s also considered an eco-friendly way of overtaking and is therefore preferred.