For a turboprop aircraft in one engine in-operative operation, why does the asymmetric (acting on one side of VT only) propeller slipstream induce a yawing moment?

I am not asking about the yawing moment created due to direct thrust which is a major part.

From : Don Stackhouse

It's really quite simple, just another example of Bernoulli's Theorem. The air in the slipstream is moving faster than the freestream air, as a direct result of the thrust-making process. If the engine on one side is shut down and the other engine is at high power, then the air behind the dead engine (including the air on that side of the vertical tail) is moving slower than the air behind the still-functioning engine, including the air on the other side of the vertical tail. The faster airflow along the live-engine side of the vertical tail therefore has less pressure than the airflow on the dead-engine side of the vertical tail. This difference in pressure is seen by the vertical tail as sideways lift, which tends to yaw the plane towards the dead engine, the same direction as the yaw caused by the asymmetric thrust.

Anything that makes air flow faster on one side of a flying surface, or slower on the other side, can induce lift. This is also why the stall speed of an airplane is usually less with the landing gear down than when the gear is retracted. The landing gear impedes the airflow under the wing, slowing it down, therefore increasing its pressure and increasing lift.

Don Stackhouse
DJ Aerotech