I'm not quite sure what forward "movement" is, but as far as "getting forward" is concerned, I can see great value in this advice for many people. The primary reason being that problems often occur when a skier's cm is too far behind their feet. Problem number 1 is that they are unable to apply pressure to the front edges of their skies without pushing themselves further into the back seat. Without proper pressure on the front edges the turns don't work; so with a cm too far back, they either don't properly pressure the front edges and slip out of their intended line, or apply the force anyway and end up pushing their cm even further back. Once too far into the back seat, the only steering they can do is tail steering. While carving turns using the tails of the skis only can be fun, it is usually not enjoyed by skiers who are not properly balanced to start off with.
Somewhat better skiers manage to get their cm above the front half of their skis by leaning forward with the upper body only; their hips are too far back and they compensate by moving their head too far ahaed. While these skiers can apply pressure to the required edges, they do not have the proper skeletal and muscular alignment to make efficient movements.
I don't know how movements can originate from the knees or ankles. While I sometimes lead with the hip, my movements usually start deep in my belly.
In the martial arts, it is sometimes helpful to think of centres. The centre of mass located a few inches behind the belly button, is often used as a centre of power (Hara), another power centre is in the centre of the chest, another centre of attention the head, often the knee is also used as a point of reference, obviously the foot as a base of support is important. Keeping these "centres" properly lined up simplifies things.
As to applying the old tip centre tail, you always have to balance the amounts of force with the response of the particular ski (and snow surface). Ideally, it seems to me, you are not making semi-circle to semi-circle, but gradually varying the radius from infinity at transition (across the hill) to maximum radius when pointed straight downhill. You need less force on the tail at the very end of your turns as you only need to maintain grip for a less tight turn than you had to maintain mid-turn. You increase pressure as needed to the tip (not too much and not too little, but just right), maximum force is probably applied mid-ski at mid turn, more force to the rear of the ski at the BEGINING OF THE END, and reduce pressure to the tail as you near the end of the end. You can be gradual about reducing the tail pressure and applying the tip pressure, but sometimes it's fun to spring off the tail just after the begining of the begining of the end, and bounce onto the tip.