Anyone who has done both alpine and XC
skiing can tell you that alpine is a lot easier to go down hill.
This is mainly due to the fact that alpine gear is more suited
for down hill travel. Its heavy nature brings the skier's center
of gravity lower, and that combined with some other design
characteristics gives the skier greater control at high speeds.
So how would a XC skier maintain control on a slope? A large
part of it is practice, but there are some techniques.
Some of these
include snowplow, parallel turn, tuck, and step turn.
snowplow(aka pizza) is a technique in
which the skier angles the skis into the snow in a backwards
V-shape. this slows down the skier and allows more control. The
parallel turn, or skid, is very similar except the skis are kept
together, and the skier rotates into a skid. When speed is
needed, the tuck is used. This significantly lowers the skier's
center of gravity, and reduces their profile, which reduces drag
from wind resistance. Finally, the step turn allows the skier to
change their direction of momentum without slowing down or
stopping. This is achieved by stepping toward the inside of the
turn with the inside ski, and pushing off with the outer one
until the turn is made.
photo from​ Nordic Fitness Sports in Hertfordshire. http://www.nordic-fitness.co.uk/cross_country.html