We discuss the optical principles behind a novel, duMond configuration, beam conditioning monochromator for high resolution X-ray diffraction. The device is capable of being switched extremely rapidly between high intensity and high resolution settings by lateral translation of the elements. It comprises two blocks of silicon each containing two beam channels oriented parallel to and at 17.65° to the (Oil) planes. In the high intensity mode, with the beam making two asymmetric reflections in each block, the angular divergence and dispersion are comparable to that from a symmetric Ge 022 device. The high resolution setting, where the beam makes four symmetric reflections, while comparable in divergence and dispersion with the 044 Ge device, exploits both σ and π polarisations. We report on the performance of the device and show how this compares with predictions using the dynamical theory of diffraction.