This is a hard one for beginners to grapple with. We are all a summation of our own past experiences and how we process and learn from those experiences, so one person's analogy that helps them understand and internalize this concept is possibly lost on another. Add in fear and you have a recipe for simple concept that becomes hard to learn. With that, here are a few analogies that may help.
If not, feel free to ignore it. Hope these don't sound too corny for you all, but anything that helps.
What happens when you are walking in slick street shoes outside along a sidewalk in the winter and you suddenly find yourself on slick ice? You quickly shift your gate from the normal human gate cycle ( which has your body mass behind one leading foot during several phases of the cycle ) to quick short steps. Why? You instinctively know that keeping your weight ( body mass ) over your feet for most of the time as you negotiate the ice patch will help give you traction on the ice. Sticking a foot forward from your body ( momentary backseat) for an initial heel strike works well when you have foot traction, not well when it's very slippery. Same as skiing a steep slippery slope; getting your body mass over your down hill edge gives you the control you're looking for--more traction. Getting in the backseat means giving up control. There is nothing safe to cling to behind you on that slippery slope. The only way to safely navigate is to cling to that downhill edge.
If you've ever hiked down a steep pitch of loose scree or muddy clay what do you do to help gain traction? For me, I turn my downhill foot sideways to my direction of travel and really concentrate on keeping my weight out over the inside edge of my downhill hiking boot to dig into the mountainside. If I were to lean back and take my weight off the leading foot, there is a good chance I'd slide feet first down the mountain. Same concept in skiing. Your skis are an extension of your feet, not some device you ride on without controlling.
Same with a set of stairs in a house. If you've ever descended a set of wood varnished stairs in slippery socks, you know to keep your weight centered over your feet and to not lean back, or your tailbone might not be too happy with you.
You wouldn't want to drive a car from the backseat even if the backseat were equipped with steering wheel, accelerator, and brakes; you want to be upfront and in control; to see what's happening up there.
Same with skiing--scan ahead to see what's coming but focus and ski the turn directly in front of you.
Compartmentalize the steep terrain. When you have a large task ahead of you that seems insurmountable, what is an oft used metaphor? "How can we possibly get it done? One step at a time." A beginner skier will often come to the top of a steep pitch and look all the way down the hill and let fear set in. They can't imagine how they are going to ski that whole steep slope without careening out of control and crashing, and getting hurt or worst. As a beginner, you are not going to ski that steep slope ahead of you like a down hill ski racer in the Olympics, you're not even going to ski it like you do the easy green runs you are now starting to carve arc to arc. You are going to survival ski it one turn at a time. Make the first turn and plan to stop. Use turn shape and some skidding to control speed; shape your turn to a stop. Of course, refer to everything else you've learned from your instructors; hands, pole plants, upper lower body separation, etc... Keep your weight on the brakes, your downhill ski as you come to a stop. Collect yourself and make the next turn. Look ahead to see what's coming, but then compartmentalize it and focus on skiing the terrain right in front of you. As you gain confidence you'll begin to link turns, but knowing that you can stop at any time, on a dime, on any pitch.