In the lesson described above, I was told the only points of tension I should concentrate on in the body are ankles and core, while everything else should be relaxed and able to absorb. ....
....My shins are on (ankle closed) and off (ankle open) the cuff while skiing. Lots of ankle ROM . . .
....it is the bottom of your foot that allows you to diagnose your fore-aft stance for yourself. That is the primary tool....
Instructors do disagree with what to do with the ankle. Try keeping it tensioned for continuous shin-tongue contact along with your core, and allow the joints above to absorb (my preference and what I teach). Then try keeping it opening and closing, losing tongue-shin contact with each turn, and absorb with all your joints. See which gives you more sense of solidity and control.
There are numerous ways to self-diagnose skiing aft.
--One is strictly keeping your underfoot pressure in a particular spot felt by the foot. Instructors disagree on this one too. Some are ball-of-foot people, others are whole foot or arch people (what I teach). Some say start a turn with underfoot pressure at the ball-of-foot and end the turn with pressure under the front of the heel. Try all three and see what the differences are for you.
--Another way is to teach yourself to perceive the under-ski pressure beneath the front half of your ski. If it's floating above the snow or lightly on it, you certainly are aft. Being able to perceive this will tell you whether ball-of-foot, whole foot/arch, or rocking fore-aft works best for you.
--Another way is to attempt to lift the tail of the inside ski while keeping its tip on the snow. You don't have to lift that tail far; a couple of inches will do. If you can't lift only the tail because the darn tip keeps lifting despite your concentrated efforts, you certainly are aft.
There are many reasons why instructors disagree about these things. Trust that what they like works for them, with their anatomy, their skis, their binding placement, their boots, their conditions, their technique and tactical choices, and the terrain they choose to ski on. There is no One Right Way.
By the way, some instructors even say keep the pressure under the back of the arch, right under the tibia. Feeling pressure here does not mean you are aft, even though the spot on the ski under the back of your arch is behind the center of the ski. If you maintain continuous shin-tongue pressure and if you can feel the pressure your shovel feels from the snow, you can then adjust where your upper body hovers over the ski to keep that shovel pressed strongly down. This is how I ski, but I don't teach it because it asks too much of the novices and lower level intermediates that I end up teaching. This process works very well on the hard snow on New England's trails. It also works well in the bumps.