PA23 multi-engine instructor loses control during a simulated engine failure demonstration on takeoff. A runway excursion and propeller damage occur.

Narrative

I am a CFI-ASEL who is training for a multi-engine add-on rating. During this dual training flight; I asked the instructor to demonstrate a simulated engine failure (to a complete stop) during the takeoff roll. The instructor lined up the airplane on the center-line and advanced the throttles to full. Shortly after the airspeed indicator 'came alive'; he retarded the left throttle to simulate an engine failure. As the airplane veered left; the instructor forgot to move his hand to the right throttle control and inadvertently retarded the left throttle even further (I did not notice this at all - he told me this afterwards). The resulting aggravated asymmetric thrust caused the airplane to depart the runway into the grassy area to the left. By then the instructor was able to retard both throttles. As we moved to the right and back on the runway; I saw a runway light fixture 'pop up' into the air. We slowed the airplane and exited at Taxiway K. A short time later; we taxied back to the ramp; shut down the engines; and noted that one blade of the right propeller assembly was scratched and slightly bent back at the tip. No other damage was noted. I believe this incident might have been prevented had we done a more thorough briefing of the maneuver. Although I explained why I wanted to see it; we did not speak in detail about how it would be performed. From my perspective; this seemed like an easy maneuver to demonstrate; so I did not feel a need to vocalize exactly what would be done. It's likely that had we at least talked about it in more detail; the instructor might not have forgotten to move his hand from the left throttle lever to the right one. It's also possible that a briefing would have reminded me that I should be carefully watching the instructor's actions; like I normally do when I'm teaching ASEL pilots. Some; perhaps most; of the reason for not briefing the maneuver probably came from a feeling of complacency since there were two experienced pilots on board. In the future I need to remember that my observations; even during training; should be 'active observations'.