1. Extention movement of legs at transition in order to push the CoM up and unweight the skis. This is done so that the skier can initiate an steered/brushed/skidded parallel turn.
2. Up moves occurs as a consequence of how we moved the body, how the skis are interacting with the snow, and how the body moves relative to the snow. Usually we refer here to a carved turn but it could be any kind of turn.
These two should not be confused. They both cause the CoM to rice in ref. to the snow. They can occour at the same time causing a very strong up move. In addition there can be rebound, virtual bump or even a real life bump adding to the push. Skilled skiers are experts on managing these up-movements and flex through the transition like in the wc montage jasp posted to smothen out the CoM flow down the hill. They use as much up-movement needed and then absorbe the rest. Like mogul skiers.
In 99 cases out of 100 when some one that knows his stuff refers to the dredded up-move they refere to case #1 here above.