The tool mates with the sparkplug well. Does not look like it will round off anywhere. I had to use an extender as my DRZ400 plug is in there pretty deep. I was able to use my torque wrench with this wrench with the use of an extender adapter.
This is a nice little tool. It isn't like the stamped versions you see from the manufacturers (well, some of them - most seem to have stopped giving these out, at least in the US). Now, it's not like you'd get in a socket set, either, but it is clearly well made.