You talking about the nose up moment
Jet or prop at high alpha (ie low speed) with a significant thrust increase (ie fistful of throttle) sees an increase in mass airflow with a change of airflow direction through the nacelle inlet or prop disc. Result is a vertical force in the plane of the prop or nacelle face which provides a nose up pitching moment component to the equation of what's going on in pitch.
This can be a particular problem on piston to turboprop conversions with higher power output engines due to moving the engine forward to keep the airframe cg under design control .. ie a bigger moment arm for the normal force to act at.
Some such mods then need to include a SAS add-on to provide artificial elevator loading - at the lowspeed end of the envelope - to fool the pilot into seeing an acceptable static stability. In its simplest form, a SAS is just a variable downspring loading in the elevator circuit to keep control forces as they should be for the pilot. Without the SAS input, and especially for the landing climb case, one can see a major reduction in static stability. Some mod designs may see the aircraft with a stick load reversal without the SAS input. It is for this reason that some POH procedures will impose a thrust increase limit for the landing climb case if the SAS is out of action .. so limiting the nose up force and the associated nose up pitching moment which, in turn affects the elevator loads felt by the pilot.