freewheel: a clutch (as on the rear wheel of a bicycle) that allows wheels to turn freely (as in coasting)
clutch: a coupling that connects or disconnects driving and driven parts of a driving mechanism
heliopause: the boundary marking the edge of the sun's influence; the boundary (roughly 100 AU from the sun) between the interplanetary medium and the interstellar medium; where the solar wind from the sun and the radiation from other stars meet