It’s an old RFC playground rule. You are allowed to switch gun positions only once per flight. No fighting. Only one pilot in the cockpit at a time. If you don’t obey the rules, you are in danger of being called “cheater cheater, pumpkin eater” by other pilots.

"Take the cylinder out of my kidneys,
The connecting rod out of my brain, my brain,
From out of my arse take the camshaft,
And assemble the engine again."