For years, I'd tried every tool I had to stop the chainring nuts from slipping when tightening, but nothing worked. This tool does the job and is cheap. If you don't have one, pick one up before you need it.
One would think one could find something to hold the nut while trying to loosen the bold on the chain ring. I tried lots of things but couldn't stop the nut from turning until the tool arrived. Then it was easy.