Like mobboss said, a valve in line is the most simple way. Some run it up to the drivers seat so they can control it. Some just have a valve where the line comes off of the stat housing. Sorry I don’t have any pics right now.
Another thing is a Bassett t-valve. When the header line comes off the stat housing, it will have a t-valve in line. The valve is a spring and ball in a pipe tee that close off water to the headers. At idle, the valve should be closed or just letting enough water come through to mist out the headers. As you accelerate, the water pressure from the jet pushes the ball open allowing more water to go through the line and out the headers.
If you look up Bassett t-valve there should be some diagrams and better explanations.
Hope I didn’t confuse you even more with my babble.
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