Let’s imagine that you wish to fly to a VDF station that is located on a track of 090M. The initial range to the VDF station is 50 nm.
QDM is the magnetic track to the station, so in this case QDM = 090M
If you fly directly along this track without drifting to the left or to the right, of it, your VDF bearing to the station will remain unchanged at 090M. But your range to the station will gradually decrease. Your QDM will change only as you pass directly over the station.
If however you drift to the right of the intended track and you take no corrective action, you will pass to the south of the station. So your magnetic bearing to the station must have decreased. To correct for this change you will need to decrease your track direction by turning to the north. This means that drifting right when tracking east causes QDM to decrease.
If you drift to the left of the intended track and take no corrective action, you will pass to the north of the station. So your magnetic bearing to the station must have increased. To correct for this change you will need to increase your track direction by turning to the south. This means that drifting right when tracking east causes QDM to increase.
Drifting will increase or decrease QDM depending on the direction of the drift and the intended track direction. The exception to this rule occurs when a crosswind requires a certain degree of drift in order to track along the intended QDM.