Classic (or Diagonal) skiing is when you slide one foot forward, the other one back, and so forth usually in a set track (about 1.5 foot wide). You wax the classic ski with a "kick" wax which is a sticky type wax that you cork in just under the area where your boot is. When you are classic skiing, you shift most your weight from one ski to the other shuffling on. When most your weight is on one ski, you want your ski to collapse and have the "kick" wax make contact with the snow so that it will "grip" the snow. Then you "kick" back with that ski and propel yourself forward. If the classic ski does not make contact with the snow (is too stiff of a ski) then when you go to "kick" back with your ski, you won't grab the snow to help propel you forward.
TESTING - What do you look for in a Skate Ski?
The test you want to do to see that you have the right ski for your weight is to stand with your classic skis on a flat surface. Stand on the skis as you would if you were going to ski (you don't need the boots on though). Take a piece of paper and have someone slide it directly under your boot area on both skis (up at least a foot in either direction of the front and back of your boot) - the paper should slide up and back easily between the ski and the floor. Then put all your weight on one ski. Slide the paper again from front to back. You want the ski to completely collapse so that the paper doesn't move at all directly under the boot area. This tells you that the ski will make direct contact with the snow (and the "kick wax"). If the paper slides easily under the boot area, the you are either to light for that ski or it is not a proper classic ski. It also means that the kick wax will not make contact with the snow when you put your weight down the kick wax will not help you propel forward.
Classic Ski waxing pocket - pdf file |