Skiing on two skis is pretty easy. Skiing on one is tougher. You get less lift with only one ski, but apparently it's a lot more fun. One thing you can do on a single ski that you can't really do with two is cut sideways really hard the way Robert is doing here.
Oh, I love shooting skiiers with the afternoon sun on their face. The light looks so good, doesn't it? :) Everything just glows.