DoubleSlash skill is the one you toggle on/off, the FlashDash skill you have to press the key to use every time you want to use it. I think it does some additional damage simply by dashing against mob (when you do it without killing mob, it works like SD for warrior except it does it only for one tile movement forward only vs SD I think moves you forward two or three tiles (?)
I don't test this beta, just saying how it worked on mir usa and eternals private server (which is pretty standard as far as I can tell) which I play now till chronicals opens live.