To perform a Dash Slide, or Dash-Cancel Momentum, dash with the Pegasus Boots then pull the control stick in the opposite direction. Doing this will cause Link to slide over a short distance before coming to a stop. If you fall of a ledge during a slide, you will conserve the momentum Link had at that point for the duration of the fall, allowing you to travel a greater distance than normal.
Instead of pulling the control stick back during a dash, hold down L to charge a dash, and then hold the stick as well, in the direction opposite to the one you're facing. This will cancel the dash as soon as it begins, effectively removing the timing factor for certain Dash Slides.
For some reason, Dash Sliding of the top of a set of stairs allows Link to cross gaps that are a bit larger, which in turns can let him access areas that are on the same level as the one he Dash Slides off, as if the stairs gave him just a little bit of extra height. This idea is used in both the House Of Gales 2F Skip and the Swamp Palace Sequence Break.