Vertical motion is the pledge of confidence and control in making carves. By bending and unbending your knees you can change the load of the board and direction of your motion quickly as well as to pass uneven slope and increase your speed in each turn!
Vertical motion is a general term and it mainly describes bending and unbending of legs in the process of running the slope. Depending on preferences and conditions of your snowboarding you should learn down-unweighting or up-unweighting. At the same time a high class snowboarder can combine these techniques while riding down the slope by choosing the most convenient one for a certain area of the slope.