Quick reaction is the name of the game...Same thing happened to me going up a hill one time, basically 2 pops and it quit. I barely had my hand by the petcock by the time the engine quit, in gear mind you. Then I had to maneuver to the side of the road, one handed, with the rear wheel locked.
Great way to start the day!
I hardly ever have to switch to reserve, but when I have, it's usually with more warning. Going uphill gives you like no reaction time.