Engine starting mystery???

[dutchy805]

A few weeks ago in Murwillumbah I helped a guy to start his Falcon which I heard cranking twice with no indication of spark. When I noticed the vehicle was a manual I told the driver to put it in first, let the clutch out and turn the key after which, I warned him, the car would move forward, which it did. I intended this action to simulate clutch starting however almost immediately after the car started rolling the engine started and off he drove. My question is - why did the engine start? What difference did being in gear and rolling make to the ignition system? Some years ago in Melbourne I did the same with a woman driving a Fiat which had stalled and blocked both lanes at peak time and I intended this action to just get her car off the road but it's engine started also.