The resistance to unobstructed flow is a termination. Close the nozzle to stop the flow, and you have a short. If you terminate video with an electrical short, then you have no video. Next, remove the nozzle, dry your self off, and you have a lot more flow than probably desired, you have un-terminated the line. In video, an un-terminated line will show much more video level than expected or desired.
Double termination is a bit more of a stretch in the water analogy.
Consider the hose an nozzle all working fine and then turn the main valve down some. The flow out of the nozzle is double restricted and the flow is reduced. The video result of double termination is a signal of much less level than expected or desired. The general rule of thumb is to terminate every video line ONCE, preferably at the end. A line or cable which is attached to a monitor should be terminated, at the monitor.