results depend on the situation: if the skier is in boots on cement, you get body moving forward. If the skier is gliding upright on a green like the coach in the video above, then you get what you see there: a bit of both. If the skier is in the middle of a very energetic WC turn or rather transition, with the knees at 90 degrees and skis off the snow, you get something else.
it's the difference between
and
On a separate note, if one would rather start the new turn already extended, blue jacket is the ticket. If not, then red pants. (orange arrows mean active effort, blue is result).
Also, red pants can't do anything about bringing his COM forward at that point in time, since there is nothing to push against (skis are off the snow)... all he can do is keep pulling the boots back. Of course the core is active in the second case as well, but not really specifically to get forward... while the hamstrings are intently used to keep the boots back in both scenarios (less in the first, as there is less leverage).
There is an issue with the blue jacket, resulting directly from not using the hamstrings much and complementing with other muscles too much: he's locking the hips, but that's for another thread...
Different situations, different actions, different outcomes...
'nuff said?