/testing and tuning/
Engines can be held at a certain RPM or load to diagnose any problems or smooth out any flat spots in the power delivery.
Not only power output is measured, boost, air fuel ratio, RPM, advance & oil temperature, etc. can also be logged.
A complete RPM range dyno session measures approximately 20 pre-defined spots one after another. The procedure takes up to 10-25 minutes after having tied down the car.