Some details on the Focus RS engine overboost function from the owners manual:
Your vehicle has an engine overboost feature to allow additional torque delivery. The overboost feature controls a variety of engine parameters to deliver additional torque. This feature widens the engine speed range of the peak torque curve, giving maximum performance while passing another vehicle or launching from a standing start.
The system allows a maximum of 20 seconds of engine overboost. After this the system reduces turbo charger boost and maximizes it at a predetermined limit. Once the turbo charger boost drops below the predetermined limit the system resets and a further 20 seconds of engine overboost is permitted.
Note: You do not need to take any action as the system allows overboost when you demand it.