What you are watching is the process of an animated shot from start to finish. It starts with a rough idea, loosely drawn (1st pass). When that 1st pass seems to work, the idea is then fleshed out further by drawing in more detail, but still drawn loosely (2nd pass).
Once that is approved, the loose drawings are then "cleaned-up", meaning the multiple loose lines are reduced down to a single line defining a shape in which it can character can be digitally colored (as seen in the final/finished version).
When is comes to animating things like a dress. The main body of the character is roughed out first and then the overlapping dress action is applied.
For this test, it required a study and knowledge of how fabric moves in the wind. The amount and degree of movement helps to determine the type of fabric. The less it reacts, it suggests that it might be made of a dense fabric, like wool. The more it reacts might suggest it is something as sheer as silk.
Anyways, I hope this was somewhat enlightening and you enjoyed it!