The physics of a passenger sitting properly in a chairlift do not actually require use of a restraining bar. If the chairlift stops suddenly (as from use of the system emergency brake), the carrier's arm connecting to the grip pivots smoothly forward—driven by the chair's inertia—and maintains friction (and seating angle) between the seat and passenger. The restraining bar is useful for children—who do not fit comfortably
into adult sized chairs In addition,
restraining bars with footrests reduce muscle fatigue from supporting
the weight of a snowboard or skis, especially during long lift rides.
That said I always put it down, because in Euroland the lifties will scream at you and even stop the bloody lift if you haven't put it down.
The thing I hate do is people who slam down the bar immidiatly, Usually parents with kids, before you even have a chance to get on the bar has already knocked of your teletubbie like appendage known as a GoPro.