The pilot was engaged in a local flight and was practising hovering at various altitudes. With the helicopter heading 320 degrees, the pilot commenced a slow descent from about 15 feet. The wind was 310 degrees at 10 knots gusting to 20 knots. While descending to about three feet, the helicopter began a slow yaw to the right through 30 degrees. The pilot elected to let the yaw continue and carry out a 360 degree pivot turn. After turning through about 100 degrees, the pilot stopped the descent by increasing power and pulling on collective. Coincidentally, the pilot reported that the helicopter then rapidly increased the rate of turn despite the application of full left anti-torque pedal. After about one turn, the pilot said he lost all control and following about another 3-4 turns, the helicopter crashed onto its left side at about 45 degrees angle of bank. During the sequence several mainrotor blade ground strikes occurred. The pilot said that he was also aware of overcontrolling on the cyclic control and pulling on collective at one stage in his efforts to control the helicopter. The investigation could not find any mechanical cause for the loss of control. Both the pilot and passenger reported that at no time did the passenger touch the controls or have his feet near the anti-torque pedals. Studies on helicopter tail rotor vortex rings indicate that these rings can form with a relative wind from 220 to 320 degrees at 10 to 25 knots, with further indications that the worst areas are around 250 and 290 degrees relative. In addition, the conditions suitable for the formation of a vortex ring are most favourable when the tail rotor is moving to the left, as in a right pedal turn. At the point where power was increased, the tail rotor was in a critical position with a relative wind from 250 degrees and strength from the left of between 10 and 20 knots. If the tail rotor was close to a vortex ring state, increased power would, through the torque effect, produce a marked right yaw. Due to the vortex ring, left pedal input would have no effect.