One of my favorite "drills" for learning extreme upper lower body separation is Pivot Slips.
Start out on a medium blue pitch or even a black if you are comfortable with it. Begin in a heel side side slip facing down the mountain. Hold both hands out in front of you clasped together like you are shooting a pistol and point straight down the run.
As you start picking up a little speed in your side slip, use just your a twisting of the hips and spine to pivot your board back and forth. Try to see if you can twist all the way around to side slip it toe side while facing down hill. Most of us cant twist that much, but explore your range of motion and see just how far you can pivot your board while remaining facing down hill. Generally, I can go through the fall line to about 45 degrees each side.
This will look and feel a little dorky, but it is an extreme example of total upper and lower body separation and getting used to the feel of it will make doing it in your regular riding feel more natural and controlled.
Once you take this concept back to normal riding, pick the steepest run you are comfortable riding and start down it, but this time keep your front shoulder generally pointed straight down the run. Now, using all of your normal skidded turn movements of twist and tilt to initiate short radius skidded turns (to control speed as your goal is to ride down a steep pitch at slow to moderate speed), throw in this pivoting movement you played with on these drills. Remember to stay nice and flexed with loose legs too.
Like anything new in snowboarding, at first this will feel like WTF, but play with it and you will slowly get a feel for the effect it is having on your board for steep terrain. Then, in your more relaxed cruising riding, using it but dialed back, will give you a little of the looseness and fluidity you are talking about.....