Strongly underwound magnetic field is observed to be correlated with the occurrence of co-rotating rarefaction regions (CRRs). We demonstrate that the phenomenon is likely caused by the combined action of (1) the motion of footpoints of open magnetic field lines on the Sun from within to outside coronal hole boundaries; and (2) the effect of solar wind shearing in rarefaction regions. We provide formulas that may be used to relate the observed underwinding of magnetic field lines in CRRs to infer the velocities of footpoints through coronal hole boundaries. The observations of strong field underwinding in CRRs generally confirms the hypothesis that the footpoints of open magnetic field lines move on the Sun through coronal holes, the sources of fast solar wind.