Nagawa ko na po kasi ito once, when I tried to help a friend whose battery was drained!

So I started my car, and removed the battery (to start hers) and the engine kept running.

It worked well without problem, pero I don't get the principle behind it!

I used to think na kasi once the engine is running, yung alternator na yung nag-susupply ng voltage sa ignition coil (so on).

But I'm not convinced and it still is a mystery to me.

Can someone please englighten this?

Thanks