Are you writing about airplanes or helicopters here? P-Factor (asymmetric propeller loading) very definitely and directly affects yaw in propeller-driven airplanes in upright or inverted flight when the propeller disk meets the relative wind at other than a right angle, as in a climb, slow flight, or even during rapid pull or push-ups (or downs). It is also one of the strong, yet little understood factors that affect pitch in knife-edge flight or at other times the fuselage is used to generate lift, as in a skidding turn.

When pitch angle increases while the propeller is producing thrust, the component of thrust perpendicular to the flightpath "lightens" the nose, effectively moving the CG back slightly, approximately as a function of the sine of the pitch angle, considering the arm at the prop hub. By "pitch angle" here, I mean the angle between the prop shaft and the flight path.

With a helicopter, in normal flight, would-be P-factor is corrected for with cyclic pitch variation. This is not the same as retreating blade stall, although it does result in momentary asymmetric lift distribution. The 90 degree "offset" in cyclic is due to the precession response of the rotating wing, unless I am mistaken. I am not an expert with helicopters.