The penstock acts as a vertical gate within the pentrough with the quantity of water flowing onto the top of the overshot waterwheel controlled by an unusual roller and chain mechanism. As the water fills the waterwheel 'buckets', the weight of the water starts the wheel rotating. The power produced by the waterwheel turns the 'pitwheel' on the waterwheel shaft to drive the 'wallower' pinion on the main horizontal shaft. The runner  Millstones are then turned by the bevel gears, stone nuts and stone spindles. The main shaft also carries a large pulley that powers an overhead shaft fitted with pulley wheels for the canvas belts driving the Aspirator in the Garner and the archimedes screw meal conveyer.

The penstock control is located inside the mill, close to the Meal Bagging area and within easy reach of the miller. The lever, when moved around the semicircular board, rotates the penstock chain drum to raise or lower the penstock in the pentrough outside. This controls the flow of water from the pentrough onto the Waterwheel which in turn controls the rotational speed of the waterwheel and of the millstones. A sluice gate is installed upstream of the pentrough to control the quantity of water, or to stop water, entering the pentrough.

The penstock and pentrough were renewed in 2014 to replace those installed in the 1970's after they had deteriorated.