With a dyno you aren't able to put a realistic load on the engine where it has to accelerate a mass, you load it down to where it stalls with a water brake then release the load slightly so the RPMs can rise; once the load is reduced the torque input to the dyno also goes down.
Mmmmm, not quite the case: the torque from the engine does not change when the acceleration starts. Maybe you meant to say the drag from the dyno goes down?
When it is at a steady RPM, the dyno load's drag (reverse torque, if you will) presented to the engine equals the engines torque; there is net zero torque, and so the system does not change speed.
When the load's drag is reduced to start the 'run', the engine torque (input torque to the dyno) does not change at that moment, and now exceeds the drag, so you end up with net 'positive' torque in the system that accelerates everything.
Yes, steady state loads take a lot of errors out, particular time related errors.