The concrete dam (380 m long and 34 m high) enlarges a glacial tarn below Moelwyn Mawr and the Craigysgafn ridge. The resulting Llyn Stwlan reservoir is the upper component of the first pumped storage hydro-electric system in the UK, which began generating in 1963.
At times of low electricity demand, water is pumped uphill from the Tanygrisiau Reservoir (behind the forested ridge in the foreground) into Llyn Stwlan. When required, the water flows back, driving turbines. I don't know whether there's a net gain in electricity, but that's not the intention; the primary purpose is temporary storage of overcapacity.