Three skiers were putting a skin track in across a 34 degree slope to gain a bench. The slope was approximately 150' tall with a bench below and a bench above. Anchors on the slope were few and sparsely strewn. The lowest skier was at the far right (climbers right) of the skin track about to kick turn and head up the next switchback. The other 2 skiers were at the top of the switchback up and to the left of the lowest skier. There was a remote fracture 30' up and right of the 2 highest skiers. The crown was 4-8" in height and approximately 60' wide. The slab had a depth (while running) up to the cuffs of the ski boots for the upper 2 skiers. The lower skier experienced a slab depth up to his waist that he was initially able to resist. The increased snow buildup in front of the lower skier eventually swept his feet from under him and he was swept approximately 20 feet down hill. The lower skier was buried up to his waist but easily able to self extricate. The slab triggered on a rock slab and slid approximately 80'.