I didn't understand how he eliminated the feedback by ensuring the mic and monitor were at the same phase... So he placed the mics one half phase from the moni

I didn't understand how he eliminated the feedback by ensuring the mic and monitor were at the same phase... So he placed the mics one half phase from the monitors so that the high frequency feedback cancels out? But so would the other sounds....? Other than placing high filter on the monitors, I can't see why his way worked