The certified flight instructor (CFI) stated that he was tasked with flying with the student the morning of the accident. He then spoke with the student's CFI and was briefed on his progress and ability. Preparation for the flight included weight and balance check, and a check of the weather. Preflight inspection of the helicopter and operational checks of the helicopter were performed using the checklist. The helicopter was taxied to a defined spot on the airport and he began teaching the student how to hover. The CFI allowed the student to control each control independently, then allowed the student to hover with corrective input from him. Approximately 15 to 20 minutes into the instructional flight while hovering, the CFI later reported recalling the helicopter moving aft followed by a violent jolt. The helicopter rolled onto the left side and caught fire. Both occupants exited the helicopter from the right side and were not injured. Inspection of the helicopter following recovery revealed the main rotor hub was fractured and the associated main rotor blade was in 2 pieces located approximately 30 and 60 yards away, respectively. The fracture surface of the main rotor hub exhibited features consistent with cantilever bending overstress, and the separated section of the main rotor blade exhibited compression wrinkles.