I've had it happen at 40-411% to stupid percents. A fun tidbit is that it actually can't happen at a percent lower than 40% but sadly not for a good reason; dash attack doesn't knock Falco over even at full power until that point.
You have to hit Falco out of an action where he's compromised his tech window if you do it to a grounded one. This basically means 20 frames after he's compressed L or R all the way down, you have a window of 20 frames to knock him over with dash attack and he can't tech. This should allude to shield, roll, sidestep, and WD as possible things that give you the okay to dash attack. However, we have a more obvious place to use this - just avoid the ground tech issue entirely by hitting him in the air. Because of where dash attack's hitbox is placed, hitting Falco out of his aerials is viable (sneaking under bair is tricky though so not sure if I recommend) but don't use this to punish landing lag because the nature of his SH aerial inputs often includes down for the dair or FF (which causes him to be instantly grounded from your dash attack, since it's so weak), and any of the shield buttons (the two most common ones can initiate tech).
A common misuse of this move is vs Falco's bair. I know with everything I said it sounds like using this to punish retreat AC bair is perfect but the AC window plus how far Falco moves away makes the hit window for this tiny. If he shields your attack then you eat a shine too. On top of that, if they simply know to hold down and shield immediately upon landing they're prepared for the dash attack on hit with the tech ready, have a shield coming if your timing is late (and then they get the free shine), and so forth. Aside from hitting Falco just as his bair's hitbox expires (he's still airborne) or something that introduces other complications there's not much you can do about it with dash attack itself. He has countermeasures. As a result, I suggest up smash or going around his bair with your FJ (get in front to soften his u-tilt) since ground stuff like dash dance grab is kind of subject to his fade on the bair IMO and the penalty for being off at all is huge.
Well, that was a tangent. Back to dash attack vs Falco. Aside from the air, the obvious places where Falco probably isn't crouching everything or compromised his tech window is when he's walking around or doing other forms of janky bait movement (usually walking or WD stuff). The fact that he can shield at any time during some of these movements may discourage you from dash attacking into grounded Falco even if it looks like you could get something juicy because you fear being shined OOS into a Fox pelt. That said, pay attention to your opponent because you might find the thing they're fishing for (on the plus, ground tech vs full walk often results in tech rolls so tech stand into shine probably won't happen). Sadly, Falco's have realized this is kind of silly so they're craftier and laser with their movement baits more intelligently (read: they do it gayer).
Other areas where you might get away with this include after he's whiffed grab. Throws require direction and that might discourage him from keeping his finger as firmly on down as usual. Any time he's about to go into the air is also fair game; if you can hit him out of his jump's startup, odds are he's worrying more about which direction he's going to jump (and what jump animation he needs for appropriate fades) rather than crouching further. FF comes at the peak to end of the jump too so that cuts the window. Finally, if you hit him during waveshines and such you can likely knock him over because of how waveshine screws with the tech window but the down input for WD and shine will likely result in a tough tech chase to hit. If they buffer the getup attack because of their shine input (or they react fast to being knocked over instantly) you can also just legitimately not have a workable situation - the getup attack hits you out of your dash attack lag.
So yeah. It's fickle. It's almost amazing but it's like a lot of Fox's attacks where it's almost amazing but there's just these glaring issues related to attack usefulness and reliability that prevent the attack from being stupid (some examples include: SDI versus dair and uair can limit their utility, u-throw doesn't combo a bunch of characters, in general his attacks are small hitboxes close to body = counterattack food, shine doesn't combo airborne opponents reliably, etc).
Closing remarks about how it works in neutral are basically that it's almost good but it's inconsistent to make sure you're accounting for how the opponent can deal with it so they don't. If it catches them off guard, it can change the situation too quickly for them to react properly. But you need to do that or else you're exacerbating Fox's fundamental weaknesses in combat and that will only take you dark places (namely Peach's dress and various characters' arms).
Well, we now know when to dash attack. So we hit one. What do?
I'm feeling generous; I'll talk about using it combos now. Because it's so weak, you can often screw with someone's tech window by dash attacking in a place where they expect u-smash or something else. Because of how tech windows work, this can on occasion lead to an unexpected knockdown; unexpected knockdowns are cute and jab reset > SHFFL uair (or grab / various other forms of gayness) on a Falco's unconscious body can seriously be a big help in keeping pace with his simpler, more reliable combo game. Depending on how you condition them, you can forgo dash attack or even make them begin to DI for it and open up other options but if I start doing that then this post won't end.
Okay so the above mostly deals with an airborne opponent exclusively so let's talk about tech chases and the rest of air combos here. Honestly, during tech chases the move is kind of underwhelming because of two things: (1) how long Fox's dash JC grab is, which is how he should be managing most techs; and (2) because he has actual attacks to hit a knocked over opponent (u-smash, aerials, shine situationally, u-tilt) and dash attack has the nasty 7% damage maximum that makes it bad vs non-tech (although if your dash attack usage can encourage non-techs you can get stuff from that too but let's not go down the conditioning road or we'll never leave here). Anyway, for similar reasons to why it flubs on SH aerial landing lag a lot, using it vs opponents during a tech chase can be dangerous and even just not productive relative to other options due to its lag and how fast it grounds someone holding down. That said, its low knockback is good for denying edgecancels on stages that aren't FOD and it can be really useful for creating a situation where their tech options are functionally slashed down. If you are reading this right, the last two paragraphs should have a good chunk of fluff for the informed members of the audience if only because we should know by now that anything can be worked into a combo.
In closing, dash attack has a decent hitbox for getting under things and its speed allows it to be used to stuff things. On hit, it pops up slightly and it obeys certain percentage rules vs FFers for when it can be used to make a knockdown and vs Peach, Sheik, and other characters with fast counterattack aerials the important percent is knowing when you can shield or dash before they get their free hit on you (since you can die). Because it is laggy, 100% stun-lock combos following its use will be hard for a variety of reasons (but mostly that it also always gives the opponent access to their non-tech option pool with an advantageous window if they get grounded by it and don't roll, which is common and can be complicated; if they tech stand they can often grab / shine / d-smash you into death). Because of how fast it is and its decent range in front, it will sometimes be the only move that will true combo off a previous move. But even in those cases the payoff of letting them land (FFer exclusive mostly, sometimes Sheik; this also assumes they have to tech) or chasing with a bigger move (or playing around their landing area; the last few suggestions relate to non-FFers, including Sheik) might give validity to another option. Dash attack in combos is complicated. This isn't Peach or Sheik.
Anyway, enough fluff. Also, this was all only about the initial hit on dash attack. The weak hit of dash attack is a whole other beast entirely with how you can use it. But this should get you started.
Won't talk about its role in edgeguarding really odd things because then this really would take forever.