@Jam, it takes a little bit of timing. I really only try to do it in cases where that's one of Falco's few options, near the edge.
The good thing about jabbing phantasm, is that jab lasts so long, there are times where you jab the invulnerable part, but falco still gets hit behind you. Even if you only get the phantasm to clank with jab, it's still good.
You can also punish phantasm in more traditionl ways: usmash, fair Falco offstage before phantasm, etc.
So when you say flick the control stick, do you mean, flick it, and hold the control stick in that direction? I mean that's what I generally do and my timing fails on it.
And someone just put the button combination step by step using the c-stick. I remember Brett posting a while back about how he always guarantees a hit after he incorporated the c-stick into the b-throw to dash attack.
Holding the control stick is probably where events don't proceed as intended. When you hold the control stick, you tend to buffer a walk instead of a dash.
Here is step by step what I do, and I get the DA a vast majority nowadays unless I really screw up.
After the bthrow:
Prepare to move when you see the kick coming out.
When the kick hits the opponent, input the dash on the control stick as fast as you can, and
release.
Slightly after moving your control stick to dash, you have to move the cstick in any direction quickly, then release.
Same as the control stick.
The result is essentially tapping on the control stick and the cstick at relatively the same time(cstick slightly afterwards). When you see the kick hit and then let go of the controller, you see the dash attack come out a little while later.
This does a few things:
1. If you mess up, you're more likely to do nothing at all (If you dtilt, it's the way you tapped the sticks)
2. It's easier to understand the buffering window if you're not holding any inputs. For me, the cue was the kick. It can be something else for you.
3. More precision since the movements can be replicated fairly easily.
4. **You're SURE it's buffered. You know it will hit and if it doesn't then it was not possible to hit them.