Although most central stars of planetary nebulae show hydrogen-rich surface abundances, a small fraction (5-20%) display an expanding hydrogen-deficient stellar atmosphere, which cannot be explained by radiation pressure. This is the so-called wind momentum problem. It is currently far from clear what are the physical mechanisms and evolutionary paths which expel the outer hydrogen layer from these carbon-oxygen degenerate pre-white dwarfs, and transforms it into a fast stellar wind. We present WiFeS/IFU observations of a carefully selected sample of planetary nebulae having hydrogen-deficient central stars. Our new results will shed light on the formation of these stars via better elucidation of their characteristics in combination with photoionization models of planetary nebulae.