2nd of 3 skier party released soft slab at top of chute and was carried approx 250 feet. Initial effort to ski to a prediscussed zone of safety helped clear skier from downhill hazards (trees/rocks), but he was unable to reach said zone. Skier was wearing an inflatable but could not get to trigger by the time decision was made to pull. Skier always had air and light and knew which way was up, but did submerge within snow for brief period. Swimming two handed backstroke uphill brought him back atop snow. Came to a rest buried thigh deep and self extricated. Lost a ski and both poles.