How you invasion moving your center of mass is perhaps different for every skier. I firmly believe saying things like “Get your hips up” is pointless crap coaching. (I say so because I was once totally guilty. Pelvic thrust my ass )
How do you get your hips up? Try to do without straightening your back leg. If you really try hard (on dry land) you can get from a broken stack to a pro skier stack without straightening your back leg but in doing so you have gone from simulating a beginner skier to Terry Winter in one move. Possible but ridiculous.
Most skiers who do not run 35 off and beyond ski with their hips further back than is ideal. For these skiers simply extending both legs moves their pelvis up and forward. This is not a yoga move. It is extremely simple bio-mechanics. I do not advocate locked knees. I promote the idea that straighter legs are a first step to a better stack.
The simple fact is that if your back knee is bent more than your front knee, your hips will be behind you and chances are your shoulders are forward to compensate. If your front knee is bent more than your back knee your hips are very forward. (this is awesome but few skiers can actually do it)
Some of the very best skiers in the world bend their front ankle forward to draw their center of mass further forward. These skiers also drive their front knee forward. (Terry Winter) This is perhaps optimal but is a totally unpractical concept to a skier who does not already have their hips at least centered. To me this is a very advanced concept and a waste of time for most skiers. Skier have to learn a basic stack first.
In my option this is the foundation of stack. It should be noted that there are a number of other factors that make it harder or easier to get and stay stacked but one thing at time.
(I am really talking about from the ball to the wakes but the mechanics are the same from the wakes to the ball)