i'd guess that they went there in the middle of the summer, got a lot of shots of where they had filmed the past winter, just of the green nature, etc.
in post, they used a program like after effects, brought both the winter and summer clip in, then did keyframe masking out one of the shots with a mask feather, so they could both have it blend with the motion of the skier and blur the two shots into one. if that makes sense...