Cytochrome b6f is a photosynthetic membrane protein acting as an electrical connection between photosystem II and photosystem I, and is a major site of proton pumping during photosynthesis.
Plastoquinol (PQH2), from photosystem II, is oxidised, releasing the two H+ to the lumen (contributing to the proton gradient). This allows bifurcated electron transport to be used, with one electron oxidising plastocyanin. The other electron waits on a c-type haem, waiting for plastoquinone to be replaced with a plastoquinol. The two electrons are transported in opposing directions, hence ‘bifurcated’. A second plastoquinol binds to cytochrome b6f, allowing a second electron to be transported to plastocyanin. 2 protons from the stroma are then bound to the plastocyanin, due to the 2- charge from the 2 electrons added.
Essentially: electrons transfer from photosystem II to plastoquinol to cytochrome to plastocyanin to photosystem I. In the process, 4 protons are moved into the lumen.