The bellows helps to smooth out power changes, and it works quite well. The PT6 is super smooth and an absolute joy to fly. Spool lag happens because it takes the combustion in the engine a little bit of time to overcome the inertia of the turbine and compressor hardware. Imagine pushing a car that ran out of gas. You push as hard as you can and at first it barely rolls at all, then a little faster, then a little faster. It gets easier as you go. That's why we don't want to unspool the engine -- that would be like letting the car come to a stop. We want to keep it rolling. Once it's in motion, it's easier to keep it in motion.
