Yeah, it might be a little tricky to find the thread in the Sheik forums, but its conveniently located in the "Reference Thread" we have here. Here's the link to the thread:
http://smashboards.com/showthread.php?t=159314
If you just want to see the video, here's the link to the YouTube video that demonstrates it
It's in Japanese, but you'll definitely see the difference.
http://youtube.com/watch?v=l7Q2gfVEf0k
Sheik's Boost Smash is at 0:38.
Basically its initiating your Dash Attack, then quickly cancelling it by Up-Smash'ing. This allows you to do a hyphen-smash with an additional slide from the initial parts of the Dash Attack. If you do it correctly, you'll be sliding really far across the stage while performing an Up-Smash (much like Snake's Mortar Slide). However, doing this with Sheik is horrendously difficult due to the extremely precise timing required to pull it off. If mastered however, it'd be extremely useful since Sheik's best KO move is her Up-Smash.
Go Sheik!!