The invention relates to a drive mechanism for a down draft vent system comprising a vertically directed, retractable open framework (4), which can be moved between an extended position and a lowered position by means of a drive motor (9), comprising at least two switches (11, 12) for switching off the drive motor (9) and at least one actuating element (18, 37) for actuating the switch (11, 12). In this arrangement at least one switch (11, 12) is actuated by means of the at least one actuating element (18, 37) depending on a force (FW) acting externally on the open framework. In addition to an end switch-off of the drive motor, this invention provides a safety function as a result of a resistive load switch-off which is effective both in the event of a resistance from below during retraction and also in the event of a resistance from above during extension of the open framework.