Plain English Meaning
A throttle setting used in training that makes a running engine act like one that has been shut down and feathered — it isn't pushing the airplane forward, but it isn't dragging it back either. This lets instructors safely simulate an engine failure without truly stopping the engine.
Full Definition
A power setting on an inoperative-engine simulation that produces neither forward thrust nor significant drag, replicating the effect of a feathered propeller on a shut-down engine. It is used during multiengine training to simulate an engine failure without actually shutting the engine down, by setting the throttle and propeller controls to a position where the operating engine produces about the same drag it would if it had been feathered.