I've got a shower head attached to the wall by a long metal covered flexible hose. The shower head stays attached to the wall so there is a long loop of hose hanging down between the wall and the actual shower head. When I turn the water off, the flow immediately stops. After a minute or two the water that was in the loop will start dripping out of the shower head on it's own fairly quickly. There's no leaking valve, so it's not new water pushing out the old. Any explanation?
Once the shower is turned off and the shower diverter lever is pushed down, excess water in the vertical shower standpipe (inside the wall) falls backward so most of the water drains into the tub through the tub spout. During this process, an airlock develops in the standpipe, trapping water in the horizontal portion of the shower arm and also in the showerhead.
Inside the showerhead near the back is a flow restrictor which may restrict air entering the system, air that would normally help to completely flush out the water once the shower diverter lever is pushed down.
From Plumbing And Showers