Purpose: Sliding Slash (SS) is faster than normal melee. Also, when you miss swing, you can quickly react to put more accurate swings.
Dash forward with weapon 1 equipped.
Click left mouse button.
This will slash your weapon.
Once you see this animation, press both weapon 2 and W keys (double tap and hold W).
If done correctly, your melee weapon will quickly sheathe itself and you'll boost yourself forward