@Matro,
What a skier does with dorsiflexion is all about getting the best possible performance from the skis. What's needed in order to get that best performance varies. How you need to dorsiflex, and when in the turns you need to do it, depends on a lot of factors, including the following:
-where your bindings are mounted on your skis
-where your body carries most of its weight (hips, chest)
-the type of turns you are intent upon making, long high speed turns, or super short ones, or something else altogether
-the kind of snow your are on/in: freshly groomed or skied off icy groomed or last night's storm snow now freshly groomed
-whether the the snow is soft and slightly deep, soft and really deep, or sticky and gloppy
-how wide the skis are
-in bumps whether the turns are round turns meandering down or zipper line straight down the bump field at high speed
For my own skiing, I determine how much to dorsiflex by paying attention to how well the shovel and tail grips the snow, and whether the resulting turn takes me where I want to go. If I'm getting the results I want, then my dorsiflexion must be working. Paying attention to this is a work in progress.
I may be hanging out on the front of the cuffs and applying serious leverage to the shovels at the start of every turn, and those turns may be coming so fast that I never get off the cuffs. Or, I may be rocking from shovel to tail with each turn, and as a result the tips and tails are giving me the grip I need to stay on the line I'm choosing. My skis may be maintaining grip on hard icy snow through the entire turn sending me into surprised pleasure where I thought I'd be skidding out.
So I pay attention to the grip tip and tail to see whether my dorsiflexion is working or not.
Skiers disagree on where "neutral" is. Some focus on ball of foot, others on the arch, others on the back of the arch/front of the heel, just below the seat of the tibia where body weight is carried. I belong to this last camp. With advanced/expert skiers, I haven't seen any huge difference in performance from one of these camps to the other. There are mysteries in skiing and this is one of them.
Being able to hold onto dorsiflexion for an entire run is good. Being able to hold onto dorsiflexion while moving upper body weight fore and aft is good too. Being able to lighten up on the tongue at will, then go back onto it at will, while thinking of all the other stuff needed is excellent. Recognizing that one set of skis needs more dorsiflexion than another to get the same good effects is another thing that's good to be able to do. All this comes with time, mileage, and paying attention. Losing focus is inevitable as one works on gaining the versatility I'm describing. It's a work in progress for many skiers. Those who have risen to supernatural skill levels do it all intuitively (not me).
Getting good at working with dorsiflexion is for most of us a moving target. And getting reliable results from dorsiflexion depends on boot fit. If boot fit is sloppy, controlling for desired results will be out of reach.