Demand of thrust is highest during T/O
Engines run at high thrust level during T/O and airspeed is low.
If an engine fails at this time, other engine will cause excessive yaw due to assymetrical thrust. Yawing causes sideslip and increase in drag. Yaw damper will react to sideslip, but not enough. At Low airspeed, control surfaces are not very effective.
Pilot is required to give rudder input to stop the sideslip. Giving rudder input for zero sideslip will require too much rudder deflection.
It may be the case that even full rudder deflection is not enough to ceter the sideslip indicator or rise in drag is too much for available thrust to handle. Acceleration and climb performance will hamper.
Airbus found a 'sideslip compromise' which overall reduces the drag by limiting rudder deflection. This is indicated by blue beta target. This improves acceleration and climb.
This is the problem of low airspeed when large rudder deflection is required and drag increases drastically. On acceleration, this problem ends, hence when flaps are retracted normal sideslip indicator resumes.