Never used one myself, but always understood to assume safety cut off mechanisms were not to be relied on, especially in the harsh conditions found in arb work, So as well as working as cautiously as has been mentioned, pay attention to your clothing and remove anything that could snag on a branch, belt loops, key chains, tool bags, back packs. Glove type has also been mentioned. Obviously remove any trip hazards.
Often what a normal day to day person thinks is a safety mechanism, really isn't. Our first chipper was a 16" Asplundh drum chipper, no fold up chute. The next one was same size and brand with a fold up chute. That is the only really safety thing I've seen put on chippers in the last 50 years. Controlled feed on chippers is a definite safety feature. The reverse bar, sort of. But, if you are that special kind of stupid person that reaches past it, you can't reach back and outside the chute, you are already gone. The two cables that hang down through the top middle of my chute look like a safety mechanism, and I'm sure that is what they are there for. Just think how you would reach back and find one, tangled in brush, with your other hand being crushed by a feed roller with knives all the way across it. The reverse bar, no matter how it's labeled, is used to reverse the feed to spit stuff back out, or just stop the feed, to let the engine build back up RPM's. It's used multiple times a day. If you ever get a chance to check out a non running/stopped chipper, reach in till you can touch the feed roller with one hand, and see if you can reach back with the other and grab the bar? I've heard of quite a few workers going through chippers. I've NEVER heard of one being saved by the pull cables or reverse bar. I'm sure it's happened, I've just never heard of it. Another thing to think about, and thinking is what safety is all about. Say you get your hand in a feed roller? Are you going to go through your decision tree to decide what to do? Lets put it into perspective. Lets just say the decision tree says, "PULL THE STOP CABLE". Your hand gets crushed, you think, "PULL", and it's up to your elbow, you think, "THE", and it's up to your shoulder, The cable is out of reach, so are you.