Extracting a broken sprinkler riser?
Sounds like what mechanics call an "easy out." It bites into the broken pipe so you can turn it to the left and unscrew the broken piece out. It shouldn't damage the threads unless you are that far down and keep trying to get a bite and just wear all the pipe away. Try tapping the tool into the broken piece to get a little better bite. I know this will probably be difficult because the line will give under you a little but worth a try. It would probably help to put something under the supply line to support it.
If it's worth doing, it's worth doing right. If you wouldn't put your name on it, it ain't done right!