butterfly valve and turbo lag
the butterfly valve is like a flap in the exhaust system that works in accordance to the revs of the engine, and adjust the sound of the exhaust for ex at low rpms and cruising on the highway it will not have a very loud throaty sound for a pleasant drive, but when you rev the motor on the track or want to feel that kick and excitement feel in the sport mode the valve/ flap raises to give a more throaty sound.
about turbo lag, i went over some reviews and first drives, and they all say that there is almost none or unnoticeable amount of it. this is because the turbo on the cla in general is a twin scroll turbo, which reduces the lag greatly. another thing to keep in my mind that i first assumed was that theyre probably using the same motor as the 250 just strapping on a bigger turbo and more pressure, but the truth it the block might be a 2.0l but its a completely different build for this kind of pressure for a daily driven car.