This letter presents a detailed experimental analysis of the electric fields and electron flow velocities at a rotational magnetopause discontinuity. It is shown for this event that the magnetoshearth plasma velocity is sharply decelerated on entry into the magnetosphere in very good agreement with a theoretical magnetohydrodynamic fit. This type of event is remarkable because the quantity E+J is negative during the magnetopause crossing. This case represents one of two types of valid solution to the MHD boundary conditions. It differs from previously published examples of reconnective magnetopause geometries in that energy is being transferred locally from kinetic energy into electromagnetic energy at the time of these measurements.