OK I did figure out how to break a tool.
I did this on purpose, it is not indicating a flaw.
What I did was ti set the tool into position and start turning the cam.
Once you get to the point where the buckets are fully compressed the center of the tool will also touch a rib in the head casting. It looks to me that that rib is there just for this purpose. IF you continue to turn the cam past this point, all of the force goes to that narrow spot on the tool and it will snap in half. If the tool did not break at this point and under these conditions, "something" else in the cam train would have to break. Either the chain or one of the sprockets. All of the engine parts that could break cost a LOT more than the tool to replace.
So it kind of works out that the tool is the weak sacrificial link to save the engine parts.
I could also see this happening if the tool was not centered properly. But I don't want to break another tool to find out.