I believe he rigged up the end of his stump with a special coupling that is connected to the throttle of the chain saw, and controls it much like a person would a prosthetic hook.
I think we all have to keep in mind that this is an 80s cult horror movie sequel. I doubt that Sam Raimi is really kicking himself over this mistake so I suggest that most of you leave it be and imagine that it all worked out one way or another. :)
If you notice, he runs it with a wire. He connects the cord on the chainsaw to his chest or something and when he puts his arm straight out, it revs up the chainsaw.
One important fact that everyone seems to be forgetting is...the whole Evil Dead series is in the genre of B-MOVIES! They are supposed to be choppy and a little bit off...That's what makes them great!!!!
I wondered about this when I saw the movie. Like the other guy, I figured that he'd probably rigged up a switch that he could push with his stump. Ash does seem to be rather handy (no pun intended ).
I figured out on Evil Dead: Fist Full of Boomstick that on a bonus video, he says he has a tube full of nicotine on his legs to the chainsaw. WARNING: If you have the game, beat it and go to the extras on the main menu and click on bonus video to hear about it. Remember on Evil Dead II how he finds the chainsaw behind the wall when he chained his girlfriend with the switch that said ON/OFF?