The certificated commercial helicopter pilot reported that he was practicing autorotations with a certificated flight instructor (CFI). The CFI stated that the first autorotation "went well," and ended with a "nice cushioned landing." During the second autorotation, the commercial pilot flared 100 feet above the ground, and applied cyclic control to cushion the landing. The helicopter "settled abruptly," impacted the taxiway firmly, and bounced. A main rotor blade struck and partially severed the tail boom. No pre-accident mechanical anomalies were noted with the helicopter. The flight instructor had accumulated 1,660 hours of total flight experience, which included 6 hours in the same make and model as the accident helicopter. The commercial pilot had accumulated 750 hours of total flight experience, which included 345 hours in the same make and model as the accident helicopter.