Starter Wiring Question

Hi All, currently trying to fix a starting problem (Fast Clicking but nothing else) and it had me thinking. It's probably simple but.....
The starter motor works on the bench, positive to the thread and spade connector.
I've refitted the starter, cleaned everything up. Positives all connected, and the red and white lead producing just over 12 V with the key in pos 4. I've not tried it tonight as I've left the exhaust off in case more fettling is needed.

My question is when you turn the key back what actually stops the engine I know we now have no power but what is it specifically?

Thanks, Wes

There is an electromagnet plunger in the tail end of the injector pump, this cuts the fuel supply when you turn the engine off.
The fast clicking you can hear is probably the starter solenoid caused by bad connects, as your starter works fine off the car. If you have cleaned all the earths and positive connections you might have fixed it.

Thanks Ian, I guessed it may be fuel related but then wondered about about mechanical fuel pumps? I need to check the earth strap from the block tonight and might add a new one then see what happens!