Can't ever seem to do it, my character (Jax) always does a full sluggish dash. Can't ever get his dash jabs fully, he'll do 1 or 2 jabs before the opponent falls, but other people do it fine. WTF.
Get the rhythm of the input down instead of trying to sync your inputs with the onscreen animation. It's a challenge but when you are in practice mode, do not rely on sight and sound, rely on rhythm. It's gets very easy once you get the hang of it. I can do nightwolf's jab carries even in extreme lag.