I don't worry about just holding the drill and keeping a reasonably mediocre speed. Not as fast as it will go full out, but a bit faster than I would hand crank. If it goes up and down a bit I don't really see an appreciable difference in crush or extraction values when brewing so I've not bothered to do any rigid testing on drill speed variation as affects the brew process outcome.
I will say that if you're not paying attention and have a tall, but narrow, hopper full of grain and you start the drill up carelessly, you can do an amazingly wonderful job of spreading several pounds of unmilled malt all over yourself, the floor, and any screaming bystanders, when the top heavy system seeks a lower potential energy arrangement.