EJL, Balancing is always an activity, even if we are just standing in our living rooms. So I find it surprizing that it is seen otherwise. Athough I would point out that many skiers seem to forget that fact. I use the idea of riding on a bus to explain balancing. If you are seated it doesn't take much to remain sitting upright. If you are standing and not hanging on to anything, it takes a lot more active balancing to remain standing. Mostly because the platform is moving beneath you. Now lets relate that to skiing. The ski are the platform and like the bus they are moving. Add to that the relative lack of friction beneath the skis and suddenly remaining standing is an even bigger challenge. If we become static like when we were sitting on the bus, balancing becomes almost impossible.
A RRX (Rialroad tracks) drill is easiest to describe as shallow arcs just out of the fall line. The drill starts with a straight run without engaging any edges. The skis should feel loose and slippery. By adding a little edge (tipping the skis) we should feel the edges engage and the loose feeling disappears. Notice that I didn't say anything about twisting the skis, just put them on edge and continue to balance on them as they start to make an arc. Since we are only working on the transition at this point, the arc that we create is small. Say twenty or thirty degrees out of the fall line at the most. When we reach that point progressively release the edges and re-engage the other set, which will bring us back across the fall line. When we can do this comfortably without skidding the skis, it's time to increase the angle we allow the skis to turn out of the fall line. This is where the drill breaks down for most skiers because they try to take the skis too far out of the fall line, too soon. Work up to it slowly. Eventually you will begin to feel how much more you need to move your body into the new turn as the skis turn more out of the fall line. You should also feel when you move too much or too quickly into the new turn. (Another point where some skiers will cheat by pivoting the skis instead of riding the edges through that phase).
Like Ray I see a lot of people struggle with what needs to hapen to create this outcome. Generally, I suggest that bending the inside leg and rotating the inside knee into the turn will cause this to happen, if you are patient. Others will suggest rolling the ankles first, then rolling the knees, and eventually the hips into the turn. Both work but IMO the syncronicity of the leg movements is more difficult to maintain if we are rolling the joints. At least during the first few attempts it is easier to bend the inside leg and let the body fall into the turn. Mind you if the skis skid there would be a need to roll the ankles over to engage the edges enough to stop that from happening. (Remember we are still on very shallow terrain so it shouldn't take much to get the edges to engage.)
The high end of the drill is to take the skis across the hill a lot more and make full turns (180 degree arcs) while making the radius smaller, or larger, than the sidecut built into the skis. Which requires a bit more momentum so the terrain used becomes more difficult. Not too steep though, the accuracy of the movements is still the major focus. I would add that these are still slow speed moves because it will make errors very obvious. (speed can hide some of these errors).
Eventually, we can ramp up the speed and turn this into dynamic carved turns of whatever size and shape you choose. Closing radius, constant radius, and even opening radius turns. Which is when I ask my students to go for huge edge angles because at that point they possess the level of skills it takes to get that far into the turns and to get back to neutral. I guess it's part of the idea I first heard as a little kid. Kennedy tasked our space program to get a man on the moon and bring him back safely. That last half being the more difficult part of the task. IMO that is also the hardest part of teaching carving and high edge angle carving. It's easy to get that far into one turn. (a J-turn maneuver) The hard part is geting back to neutral and setting up for the next turn.