Observations of 'supersonic' O+ outflows by DE-1 in the polar magnetosphere are interpreted as a velocity-filter effect resulting from the transport of these low-energy ions from a horizontally narrow source associated with the polar cleft topside ionosphere into the polar cap magnetosphere. Under certain simplifying circumstances it is shown that the 'Mach number' for the flow is essentially a geometrically determined quantity, approximately the ratio of the horizontal distance of the observation location to the source over the horizontal width of the source. A recently developed two-dimensional kinetic model of ion transport in the polar magnetosphere is used to calculate distribution functions and the associated bulk flow parameters, and these calculated quantities are compared with measured flow parameters from DE-1.