If it is a round steel button below and to the left of the throttle, then that is the shutoff. For it to work on mine, the throttle has to be closed all the way. When you open the throttle to start it, it pops back in.
That is exactly what it is then, I kinda played with it last night after I wrote the last message and see how it pops off when the hand throttle is engaged. I did not try it running though as my machine is apart right now for paint, lots of sanding going on about now!