The spools that I have seen with notches usually have the new thread end under the paper label at end. A pin easily lifts enough of that paper label to get a hold of the thread. Then when I put the spool away, I always put the thread end back into the notch. It is there to keep the thread from unwinding in storage, and it works.
Most times, it is hard to see the notch to get the thread back into it. So I mark a tiny line on both sides of the notch with a black permanent marker. Works like a charm to find the notch.