The student was on a dual flight, reviewing maneuvers & procedures to revalidate an expired CFI certificate. An exam of his log revealed that on the last 2 flights, he occupied the left pilot seat & practiced teaching quick stops, hovering autorotations, low rotor RPM recovery & throttle cuts. Witnesses said the helicopter had been working the area about 20 min near a hill top. They said the helicopter was hovering 'at 400' AGL' when the engine noise suddenly 'died off' & then they heard a loud 'pop.' They saw parts falling off the helicopter as it fell. An exam of the wreckage revealed a main rotor blade entered the left side of the cockpit. No preimpact system malfunction was found. Toxicology tests showed a trace of marijuana metabolite in the instructor's urine, but a toxicologist concluded there was no 'current influence' of the drug. A trace of cocaine metabolite was detected in the student's urine. The Safety Board found that there was insufficient evidence to conclude that drugs had affected either pilot's ability to operate the aircraft.