When the flooding tide comes in, it pushes open one-way gates (the 'sea gates' or 'sea hatches') at the other end of the dam from the Mill, and fills up the 3km stretch of the river upstream of the dam, just as if the dam and Mill were not there. This 3km stretch of tidal river is our millpond, and when it is full (at high tide), the water in it is about 95 percent sea water and 5 percent fresh river water. The mill cannot work at this point for two reasons: first, the tide is also high on the Mill side so there is no flow of water, and secondly the waterwheel is largely underwater itself, and waterwheels don't work underwater (too much drag).
When the tide turns and starts to go out again (the ebbing tide), the water on the mill-side (i.e. the seaward side) of the dam drops, gradually uncovering the waterwheel, but the sea gates close, trapping the water in the millpond, so the level in the millpond stays at the high tide level (which varies from day to day). There is a sluice gate blocking the channel through the dam from the millpond to the waterwheel, and at this point this gate is closed.
When the tide has dropped to the point where it is half-way down the waterwheel, if the sluice gate is opened, water flows through from the pond (still at high tide level) and starts to turn the wheel. However, the lower half of the wheel is still underwater, which means there is a lot of drag, and the wheel will only turn slowly. At Eling we run it at this point if visitors are here at that time so that they can at least see everything moving, but the flour made is too fine (the slower the mill turns, the finer the flour), so the Mill wasn't used at this point in the old days when it was just a working mill rather than a visitor attraction.
Proper milling starts when the tide falls to a point completely below the bottom of the waterwheel. When this happens, the sluice gate is raised (only a few centimetres), and the jet of water from the millpond strikes the lower blades of the waterwheel, spinning it round. The bottom of the wheel is well above low tide level, so we work from when the ebbing tide gets to the bottom of the wheel, through the time that the tide continues out to low water, on as the flooding tide starts to come back up, until the rising tide reaches the bottom of the waterwheel and starts to interfere with the running, slowing it down again. This gives 5 hours of proper running, making good flour, each tide. There are two tides a day, so the Mill can work properly for 10 hours a day, in two 5 hour shifts, each shift starting 12 and a half hours after the start of the last one.
For the sake of visitors, we can run the Mill slowly as the tide rises up the waterwheel, until it gets to the axle, at which point there is too much drag to run at all. Again, the Mill would not have been used like that in the old days as the flour is too fine when running at these slow speeds.
Eventually the flooding tide reaches a height where its level is higher than that of the remaining water in the millpond, the pressure of the tide forces the sea gates open, the millpond begins to refill as the tide continues to rise, and the cycle repeats.