i just took a good look at this. THIS is the reason chain tools break most of the time. the proper way to use it is to have the back of the chain up against the solid back end of the tool. the chain goes over that ridge, so it's actually inside of it, to hold it in place. (see thumbnail.)
if you do it as pictured, the ridge breaks off because of the force applied to it.
our problem, and the reason Park makes 10 different kind of chain tools, is we have two different sized chains on our bikes, and the heavier motorcycle chain won't fit in a standard breaker properly.
but for your bicycle chain, make sure it's lined up with that slot inside the chain, and the chain up against the back, and it'll last a lot longer.
(not trying to step on your toes, paul, just trying to help out...)