Without a governor or manual adjustment, the engine will try to compensate for the load by reducing the speed, this normallyresults in increased torque, however like you mentioned, if the patch is too dense in the absence of either governor or manual control, it will stall until you move it away from the patch and then it resumes work with a sharp increase in speed. The latter point however is the main reason why lawn mowers have governors, to ensure that no matter the reduction in load, it doesn't run above a certain speed limit.