Once upon a time the spark came from a magneto. There was no ignition switch, only a kill switch. The switch shorted the magneto to ground.
More modern. The kill switch is in line with the hot wyre coming from the key.
On, it allows the volts to flow to the ignition system. Off, it disconnects the flow. Get a stand alone switch to mount on the bar, that you can reach with your thumb. Use a switch, not a push button.