The ignition coil has two primary connections, one is connected to each side of the relay coil.
The diode will stop it working. Read my description of the 31st March. To get a spark, the current in the ignition coil has to stop very quickly. The diode lets the current continue.
If you want a physical analogy, imagine how you would produce a large force with a hammer. You are not strong enough to produce a big force on your own, so you use your strength to accelerate the hammer. When it is going fast, you swing the hammer into something solid. The big force comes when you stop the hammer, because there is a large deceleration of the hammer. If you swing the hammer and do not hit anything, there is no big force, because the hammer does not decelerate quickly.
In the coil, the current builds up slowly with the 6V supply, like you hammer gets faster over about 1/2 second as you accelerate it.
When the contacts open, the current has nowhere to go so stops quickly. There is a big voltage created. That is like when the hammer hits something and stops very quickly. A big force is created.
If you add a diode the current slows down slowly. There is very little voltage created. That is like when the hammer misses and swings free. There is little force and it takes longer to stop.
I hope this helps.