More on the propwash/ yaw subject (or propwash/ yawn, some of you might say):
When the engines are producing thrust each engine is generating a spiral airflow, with the upper prop blades generating an overall lateral flow in one direction (right to left in the case of the Vanguard/ Tyne config) and the lower blades the opposite direction. The upper lateral flow will clearly impinge on the right rear fuselage and tail more so than the lower flow, imparting a yawing motion (to the right on the Vanguard), requiring a left rudder input to cancel.
When reduced to flight idle thrust the lateral flow from the props will disappear, meaning that the left rudder input countering the approach thrust throttle setting will now cause a yaw to the left, as Scotbill and Meikleour described.
Another factor (contd. p94)