Archived KimimaruSmash video from YouTube. Original description mostly intact:

This is a commentated video tutorial for Mario and Luigi's Extended Up + B in Super Smash Bros.

Some additional notes: -Mostly any range up or down works when performing the back input (Ex. you can hold down and back and still get it), as long as back is held far enough -If you turn around, you held back too long -If you get a normal Up + B, you did it too late

Enjoy!