Could be completely wrong, but from memory the dip between the peaks is actually a deceleration spot to get past a speed where there could be a surge or stall. It's very much the same on all the family from experience with 18,20,20B,20R,30 and 47. Fast and hot is best for the engine.
Can't remember if the V-slot duplex nozzle really is changing during the start cycle and somehow doubt that this is really responsible for the start peaks. Methinks it's the start notch valve (Bendix).
Any blackhanders here with more recent to avoid pilot conjecture?
Can't imagine why this would ever be in a BGT exam - although then again!