According to the flight instructor, he gave the pilot under instruction (PUI) a simulated engine failure after takeoff when the helicopter reached approximately 50 knots. The PUI pulled the cyclic aft and increased collective. The flight instructor joined the PUI on the controls to prevent him from pulling too much collective and to lower the helicopter's nose to a level attitude. Both pilots were on the controls as the collective was increased to cushion the landing. The helicopter landed on the paved runway's centerline, and as it slid across what the pilots described as an uneven surface, it began to porpoise. The flight instructor lowered the collective to slow the slide and heard a noise, then the helicopter began vibrate and turned 220 degrees to the right before coming to a stop. During the slide, a main rotor blade cut off the tail boom. Neither pilot reported any preexisting mechanical anomalies that would have precluded normal operation.