When striding you want to swing your leg through with the hip and land on a flat foot.
You dont want to pick up your knees like you are walking. By doing this you transfer the energy of your push forward to the ski in the track.
By doing this drill on pavement, the slapping sound is amplified so the CAT skier can work on swinging their hip through and landing on a flat, forward flowing foot. Within minutes skiers learn how to ski and not walk on skis.