Short answer: You need to have an enemy in range for it to trigger.
Long answer: I initially had the same problem you did, since I bought this skill right after emerging from the starter dungeon and didn't have anything to use it on. Tapping the button (left mouse for me on PC) after the first shadow strike doesn't seem to do anything unless there is actually an enemy to attack. But shortly thereafter, when fighting 4 wolves, I was able to tap the button after the first strike and it kept rushing me back and forth.
I'm not sure how it picks which guy will be your next target when you chain another attack into it, but it does seem to need an actual enemy to be there to do it. No aiming on your part is required.
Having used this a lot since I originally answered, you don't even need multiple enemies there for this to work. It will trigger on a single enemy. But you do need at least one.