As you ride, imaging your body going straight down the hill while your board is making shallow s turns underneath you. At the outside of every turn, your board is at it's farthest point from you and your legs will be fully extended. As the board crosses underneath you, it will be at it's closest and will require you to flex your legs. This is dynamic riding known as cross under turns because now the board is starting to turn more independent of your entire body. A little bit of swivel in the waist will take place as well.
That is the goal for dynamic cross under turns. To start getting you there, do some very basic and simple exercises while you are riding. First pick a spot where you have a little more room to make completed turns. That is allowing the board to be completely across the fall line before making your next turn. As you make these linked turns, start playing with this flexing and extending. Before you initiate each turn, flex down to your 8 position and enter the turn. Ad you progress through each turn, gradually extend to your 2 position and pay attention to your timing. You want to reach maximum extension as the board completes each turn so speed up or slow down the movement to coincide with your turn. Drop back down to make the next turn and repeat the process.
An extension through the control phase of a turn increases edge pressure and helps make for a more powerful turn with less chance of loosing your edge hold. In really steep terrain, dynamic ridiculous is a must to maintain edge hold. On mellow terrain, it is just plain more fun and looks so much better. As you work with this, do these movements faster and tighter and you will really behind to feel this dynamic quality to your riding.
Hey Snowolf - is this part here essentially an introduction to what you call "down un-weighting"? I've been looking for some technical specifics of how to properly perform it, and this looks like a good exercise.