slidetossing, glidetossing, whatever its called, is where you shield, c-stick toss an item, then (while still shielding) hit left or right in rapid succession to cancel out the first few frames of a roll. You can do it while running.
Think of it as a half-wavedash after throwing an item.
It only works for character items, like Diddy's bananas, Link's bombs, Rob's spinning top, because those are the only items that can be thrown via C-stick
If you use an alternate setup to throw it, it works for all items. I'm just too c-stick reliant to change it to something else.