The Bristol Britannia with its Proteus engines had a prop brake to prevent windmilling on the ground. When starting engines in a tailwind,you would go and hold the prop to prevent backwards rotation after brake release. The Proteus was sensitive to wind up the kilt and could be hard to start. The electric starter turned the engine compressor and would wind up but the free turbine driven prop could be held. At light-off which was audible as a thud, you would feel the prop pull out of your hand and you could amble away to the next engine to be started. You could see the pax noses pressed against the windows looking down at you as they wondered what was going on.
I agree the spiral paint on the spinner is for ground crews. It is harder than you think to determine which engine is running on a busy ramp.