It's very easily replicated, much more easily than a true hammerhead. At any time when your airplane is heading directly upwards, put the stick fully back and to one side and give full opposite rudder. Hold it there until your airplane stops ascending and begins descending. Power remains at full throughout the maneuver.
I would have held it in there for longer, but I was at very low altitude. Ordinarily, I allow the tumble to develop into a normal spin as the airplane starts decending. Note that how fast you are going when you initiate the maneuver determines how flat the spin is and several other aesthetic factors. As I said, I did it very sloppily. A good aerobatic pilot could make this much more impressive and also exit the maneuver in the same direction in which he entered it.