Ok, believe it or not, I'm not a used car salesman, but I have give a disclaimer up front. Disclaimer: It works for me, but might mot work for you.
I have not done this in corn. I have done it many times in wheat, grass and small seed legumes. Each of these crops use a rotor speed of about 600 to 1000 rpm. On a 1680, that would run in high rotor gear. Example: Combining wheat, 120- 150 bpa. Rotor pull down, 50 -70 rpm from empty speed. Second gear in transmission. I will move my left hand over to the hydro lever, and my right hand on the throttle. While continuing foward motion at the load I run at, in one quick motion I push down on both brakes, pull back on throttle to stop position, and push foward on hydro lever. Do not turn off switches till everything is stopped. Once all still, turn off swithes, pull back hydro and restart engine. let engine cool, then shut off engine. Now you can remove shields and panels to see very closly what the inside of the machine was doing while operating. You can go all the way to the header and look for shatter loss, inside the feederhouse to look for bunching, inside the transition cone to see how it's feeding. And everything else you can think of. You can see on the conveyor auger bed how the material is distributed and if there is a heavy spot, you'll see where it starts. You can check the tailings for content and volume. In corn, you should be able to see the condition of the cobs as they travel to the rear of the cage. Then to restart, again, on my machine, with me operating it, I will put the rotor in low. Sometimes I have to slightly rock it to change gears. Then I will start engine, move throttle to about 1_3, and engage seperator. At the same time the seperator engages, I push throttle to full. let machine clear then engage header. Now having said that, I will say, it may take a couple times to get the hang of it. If you think the machine is almost choked, then put the rotor in neutral and only start the seperator. In fact, if you have a chopper, this may be the way to go. After machine is clear, then put rotor in gear and continue. DONT engage header utill you have the rotor back in gear. In all the years of running, I have never snapped a rotor or seperator belt. I have only worn them out. I do this quick kill every year several times in every crop. This way I dont spend much time guessing where any problems start and what adjustments to make. It has helped me determine where the shortcomings are on the machine and then I can modify accordingly. If you question the abusive nature of this procedure, keep in mind, that when you pull back on the throttle, you are shutting off the full. The engine is simply being shut off at an accelerated rate. If you slug the machine, the throttle is still full on. Just make sure you restart and cool the engine. Upon clearing the machine, in your owners manual, it even says to run throttle to full, then engage seperator. Although I dont do it that way, the factory says you can. Now I probably forgot a few details, so if you have questions, please ask. And if you have more to add to what I do, please post comments. I hope this helps some. It is without any doubt, the most valuable and informative method of diagnosing a problem you can do.