Intermediate Skiers of all ability levels should learn how to deal with the forces that act upon them as they ski. Sound complex? It's not.
Your ankles are the key. Think of them as doors: If you're standing upright, they're "open. If you're in a seated position, they collapse, and look "closed. First, make turns keeping your ankles open. Then ski only in the closed position. Neither position will feel natural. Next, try opening your ankles as you start your turn and gradually closing them as you continue through the turn. This technique will allow you to handle a wider range of conditions and terrain.
—David Welch, Ski Sundown, Conn.