A method is provided for controlling an automatic transmission unit including at least a transmission with different selectable gear ratios and a clutch and being associated with a propulsion unit for a vehicle. The method includes operating the vehicle at a prevailing vehicle speed (v) in a downslope with the automatic transmission unit in a neutral gear or disengaged from the propulsion unit; operating the propulsion unit in an idle mode of operation; controlling the automatic transmission unit so as to select a gear which is a relatively low gear with regard to the prevailing vehicle speed (v), while maintaining the automatic transmission unit in a neutral gear or disengaged from the propulsion unit; and engaging the clutch to be slipping between the propulsion unit and the automatic transmission unit in order to adapt the rotational speed of the propulsion unit to the gear ratio of the relatively low gear.