The issue you are asking about is how to get not-aft, right? It's important to keep in mind why we need to be not-aft. The point is to get the shovel to grip the snow. There's no other reason. If the shovel is sliding lightly on the snow while the tail is gripping, the skier has little control over the turn. That's the functional definition of "aft" or "back seat." Being not-aft is particularly important at the start of a turn, since the shovel is the leading part of the ski; it gets there first, it needs to take its job seriously and get a grip.
While paying attention to the shovel gripping, we should not forget the tail. The tail should grip too. You paid for a whole ski; use the whole thing. A tail that is light can easily swish around at the top of the turn, causing the shovel to be loosened and let go as it points downhill. A light non-gripping tail at the top encourages a pivot entry. If it's still light at the bottom of the turn that tail will have a tendency to skid out. Neither of those options is very helpful in getting your ski to take you where you want to go.
How does one weight the tail while getting the shovel to grip the snow at the very start of a turn? Lots of things don't work. Folding forward at the waist doesn't work. Shooting both arms forward a la Night of the Living Dead doesn't work. Bending the knees in an attempt to press into the boot cuffs at the top of the turn doesn't work. Standing on tippy-toes to press down on the ball-of-foots does press the shovel downward, but it lightens the tail, so that only works half-way.
Matt Boyd, one of the PSIA National Team members, did a clinic at the mountain where I work one year where he dealt with this issue of how to gain purchase with the shovel and the tail at the same time. We were all inside for this training session. Matt had a ski with an empty ski boot clicked into its binding. He got two volunteer instructors to hold the ski up for everyone to see, one supporting the shovel and another supporting the tail. Then he took his hand and stuffed it into the front of the boot, pressing down on the area where the ball-of-foot would be. He asked the two volunteers to try to lift their parts of the ski. The shovel wouldn't lift, but the tail did.
Then he pressed the ball-of-foot down again with his hand as before, and with his other hand pressed forward into the front of the boot cuff. Same result; the shovel was "pressured" downward and wouldn't budge, but the tail easily lifted.
Then he took a broom handle and stuck in down into the boot, pressing its end downward where the heel of the foot would be. He tilted the whole broom handle forward so it pressed into the front of the cuff at the same time. When the two volunteers tried to lift their ends of the ski, neither was successful. Both the shovel and the tail were "pressured" downwards. The heel pressure came from the end of the broomstick, and the shovel pressure came from the front-of-cuff pressure. Note: there was NO ball-of-foot pressure; nothing was in the boot in that area. Worth repeating: no ball-of-foot pressure at all.
His point: you can give both ends of the ski a chance to grip the snow if you stand on the rear of your foot (specifically back of the arch/front of the heel according to many) while successfully pressing forward into the front of the boot cuff with your shin. Doing that involves "closing" the ankle, which moves the entire body from the ankle up forward -- all that
weight causes the lower leg to press into the boot cuff which keeps the shovel weighted.
So do that. Bend forward at the ankle so your body tilts forward from the ankle upwards, and stand on the back of your arch/front of your heel (!!). Actually, your tibia, the big bone in your lower leg, points straight down onto the intersection of your heel and your arch. Stand on the spot directly below the tibia.
Some people say "back-of-arch" is where you should stand. Others say balance "over the arch." That's because no one wants to say stand on the heel because that quite justifiably brings up visions of back-seat skiing. Oh wait, I think
@Josh Matta said it the other day. There you go!!!
I think Matt said press the heel firmly downward. How about this: stand on the foot part that's directly below your tibia.
And bend forward at the ankles. Different side of the same coin for that ankle part: pull your feet back and hold them back. This will bend the ankles forward. Bending forward at the ankle works for some, while pulling the feet back works for others. Different cues work for different folks, thus all the variations.
Matt's demonstration does call into question lightening the heels in order to stand on the balls-of-the-feet. I'm an adult learner, so I remember that when I discarded standing on the BOFs and started standing the way Matt demonstrated with that broom, my skiing improved immensely. It wasn't that long ago.
Here's the image I like to borrow from Paul Lorenz. He is not standing on his tippy-toes; his heels are firmly planted. His shins are pressing all that weight above them into the fronts of the boot cuffs.
from:
https://snowbrains.com/wp-content/uploads/2013/07/Stance-SnowBrains.jpg