The box tag is probably one of the single most important aspects of melee for pro competitive play. The entire reason Mango won pound 3 is because of the box tag. If you can pull it off (with hundreds of hours of practice) it can give you a huge advantage in competitive play. It serves as an huge mind game tactic while also adding an extra hit box above your character (thus the "box tag" short form of "hit box tag"). If you watch any of mangos videos from pound 3 you can see examples of the box tag in use. Now that its confirmed for brawl I'll have to start practicing this technique.
If you want to learn the basics of how the box tag works, you'll have to study advanced physics for game programming, along with having a deep understanding of how hit boxes and collision detection works in brawl. The box has a priority that can be determined by this formula:
P = (TL-(p))*n
Where
P = priority
T = tier position of your character. eg. fox = 1, falco = 2
L = length of the tag (some pros can get it up to 4 chars long, most people do 2... i can get 3 sometimes)
p = your controller slot, 1,2,3 or 4
n = number of players in the match
Because of the properties of the box tag, as shwon by the forumla, the lower your character is on the tier list, the more effective it is to a certain limit. Jigglypuff gets the most out of this, any lower tier spots give equal T values as jiggs. This is how mang0 effectively used his jigglypuff against M2K and other pro players at pound 3. If you look closely in those videos, mango is also in controller slot 1, to make sure he gets the minimum "p" value for maximum priority.
The damage that the box tag gives is 1.253034 times the priority.
Heres the box tag in action at pound 3:
http://www.youtube.com/watch?v=LqkF6F0561o
http://www.youtube.com/watch?v=xdT8Hh8fApA
http://www.youtube.com/watch?v=MKC92LeWfPw <-- hard to see, but its there
