Well since I don't have a manual on me right now (I remember reading it a some point somewhere but have forgotten now), I'll think about it logically. The thicker the clutch is, the farther forward the fingers of the pressure plate will be. So the opposite would be the case for a thinner clutch, farther backwards, along with the release bearing and fork being farther backwards.

So the "notch" is just the front two inches or so of the slave rod being of a thicker diameter, and the back portion being a thinner diameter. The tool slides over the thinner part, and won't slide over the thicker part. Eventually the clutch gets thin enough to push the rod back far enough for the thicker part to be the exposed part in front of the slot for the tool. So when you can't slide the tool in it's worn.