An intuitive picture of pair breaking by a vibrating wire is developed, which makes dear how quasi-particles can escape from the wire at critical velocities the Landau critical velocity νL. Assuming strong gap suppression at the wire surface, a critical velocity of is obtained, for ideal dipole flow around a wire of circular cross-section. In addition, a further transition to enhanced quasi-particle emission is predicted to occur at a velocity More generally, for non-dipole flow, and where at αl characterizes the local superflow near the surface of the wire.