haha thanks man ^^ well.....
to do the SWD, you have to do the first zip of your up-b to the floor, then the second zip horizontally across the floor. if the 2nd zip is interrupted with a move of equal priority as the up-b, then the 2 moves will parry, and will result with the same momentum/speed as the up-b.
in my video, the first time i did it was off of YL's boomerang. the second time i used my own thunder jolt which was reflected by fox
it's a lot easier to do it off of projectiles, but you can do it off pretty much any move with a low priority, such as most characters jabs and a lot of character's tilts. it's the easiest to do off of projectiles, such as links/YL's boomerang, thunder jolts, mario/luigi's fireball, uncharged shadow balls/charged shots, pills, etc etc