You can apply any load you like on the dyno so as you increase the throttle, you increase the load to keep the RPM the same. Most videos show "shoot out" mode, where the load increase is held back a bit to allow the revs to rise. It's also possible to run at any throttle and alter the dyno load to allow the engine to increase or decrease rpm. This allows the tuner to check fuelling and ignition under any load condition.