Sure thing!

Basically, just time the dash so that your target dies (from the projectile preceding the dash) right before the dash is able to deal any damage to the target. If the dash inflicts no damage, it is not sent into Cooldown.

It's mostly trial and error until you're able to get it with muscle memory and also able to judge the minor adjustments you need to make in certain places (IE: in Tam Tara, no matter how hard I tried, I couldn't consistently get my dash cancelled off of RDM's Jolt on the first two enemies. So in my personal best run, I used Scatter instead).

A: If your Dash is sent into cooldown, it means you dashed too soon, so trying delaying it just slightly on your next attempt. B: If the Dash looks like it was cut short, or if it didn't come out at all, then you dashed too late, so go just slightly faster on your next attempt. C: In a perfect situation, you'll travel the full distance of the dash (and may even hear the sound effect of the Dash's hit), but no damage is inflicted and the CD is cancelled. But if you're able to consistently get more than half of the dash's distance for free, you're doing pretty good.

When practicing, you can stand still after you throw the projectile or even run backwards slightly to give yourself more time to focus on the dash. Once you get consistent, you'll want to be able to do it while running forward.

And a few other rules of thumb:

• It's best not to use the technique on enemies that are aggro'ed / approaching you. Since they're closing distance between the use of your projectile and the use of your dash, you're both losing out on potential distance traveled and also just making it harder on yourself to time correctly. So always just use it on an unaggro'ed add, then keep moving
• For shorter ranged projectiles/dashes (IE: Tanks), it probably isn't efficient to use this technique during a Sprint. If we're being nitpicky, a dash is technically always less efficient when used during Sprint, but in this case you might actually be losing distance unless performed perfectly
• Still experimenting with this, but you can sometimes clip around an invisible corner with a dash (other times you just dash in place and it feels bad). In this case, using it during a Sprint is probably a gain even for the shorter ranged projectiles/dashes.
• For Dashes with Charges, consider spending the first charge normally so that the cooldown is at least ticking (once optimized; until then, it's probably better to save that second charge as an "oh shit I messed up but I can still keep going" safety net)

Now for class-specific tips:

WAR / GUN / DRG - Haven't leveled these enough to be able to try them out yet. Sorry!

SAM - Casting Enpi gives enough gauge for your dash. The dash's damage comes pretty late, making it much easier to time, and it neither consumes Gauge nor CD upon proper execution. This, combined with Enpi's range being longer than other Melee Classes', makes Samurai actually really easy to work with. The drawback is that Enpi's raw potency is relatively low so it's a little harder to one shot your target if you're not maxed out. To compensate though, Samurai can buff its damage by 10% with Jinpu and can also use excess gauge on your Jump-Back skill to power up your next Enpi. SAM honestly has a lot more potential than I thought.

PLD - Shield Toss into Dash is pretty short range, but still speeds you up as long as you aren't Sprinting. It's felt most consistent when aiming to synchronize the cast of my dash with the "thonk" from the shield toss connecting. At Lv78, Requiescat's buff (removes the cast time on Spells while it's active) makes Holy Spirit a viable option as it 1: does WAY more damage (so can one-shot higher hp mobs) and 2: has significantly more range. Mashing the Dash after a max-range Holy Spirit seems impossible to time wrong, so long as you're able to even get into Dash Range before the target is dead. The only caveat is that Requiescat needs to hit a target (at Melee Range) to get activated, but if you're able to plan out your uses ahead of time that accommodates for this, I feel like this Class could make good run times.

DRK - Unmend into Dash is terribly hard to time right (by comparison), which I blame both the Dash for (the damage seems to come way sooner than the animation implies) and Unmend (the damage comes very late, so by the time you'd use the dash you've closed most of the distance already). Abyssal Drain felt much easier but is on a 60s CD. Despite this, DRK can still at least pull it off and their Salted Earth is still surprisingly handy in places like Sastasha when you have to interact with objects and a bunch of mobs are chasing you. As far as the execution is concerned -- you can damn near wait to see the damage numbers float up before activating the dash, and the margin of error between falling out of the apex of your jump vs landing the jump without dealing the damage seems pretty slim compared to other classes.

RDM - Casting Max Range Jolt and then mashing Corps-a-corps feels, in some places, easy and consistent. Other times, for some unknown reason (IE: the first two enemies in Tam Tara), you have to delay the dash a little bit. In cases like these, Swiftcasting a different spell may change the timing a little bit, so just experiment with which one you feel most consistent with. This technique is also possible with Contre Sixte, Fleche, and Enhanced Reprise, meaning Red Mage is actually way faster than I previously thought if you're able to master the timing for each one of these projectiles. With this in mind, it might actually still be the fastest class for sub-50 dungeons provided you're able to use everything to its fullest.

If you want (and once the Shadowbringers hype slows down), I can try to record some videos of each class's Goldilocks timings ("this is too slow" vs "this is too fast" vs "this is just right") since otherwise it's just trial and error using those ABC visual indicators listed above.

@lunamilegaming Not necessarily. I'm thinking every class that has 1: a projectile and 2: a damage-dealing dash will be able to do the RDM Glitch as well. Below is my earliest attempts (literally: today) of trying it with PLD and it's definitely possible, I'm just obv not as consistent with the timing since I didn't spend the time practicing this one (0:29 and 0:56 were the best executed ones, the rest were probably more of a time loss but at least didn't spend a charge)

I can't actually confirm that all classes can do it since I don't have them leveled, though. If they can though, then WAR, DRK, GNB, possibly SAM (I think theirs costs gauge which may have different stipulations) and possibly DRG (the non-"return to your original spot" jumps might work just like the dashes, but also might be harder to time) can probably beat RDM times and compete with Lv80 NIN times since they 1: don't rely on Swiftcast and 2: never have to Hardcast period on big mobs and Bosses and such. This is both assuming it's possible for all of these and that it's mastered at least to the point of being able to consistently do it across the span of a low-level dungeon. BLU is kind of the exception (no surprise) because, while they do have both a projectile and a damage-dealing dash in Flying Frenzy, it isn't insta-cast so it kinda loses its flair (it is, however, the lowest CD dash in the game, so maybe when it's raised to Lv60 it'll have something else to play with?).

DNC and BRD can still prolly keep up by quickly clearing trash and having Peleton up semi-permanently, while also having dashes unlike MCH. I still think Lv80 NIN will be the fastest period by spamming Shukuchi resets, and as for Healers and SMN/BLM, well... no, I don't think there's a way for them to take a World Record for Low Level Solo runs. They can still go for WRs, but maybe not in that category (just like going RDM for an On-Content Raid WR is sort of counter-intuitive to your goals of dealing the most amount of damage in the shortest amount of time and having no deaths).

I eventually plan on leveling those other classes to see if they can use the glitch as well, but for now it's certain that at least 2 of them can (RDM/PLD).

Which glitch? If you're referring to this one, then it is most certainly still in the game:

As for NIN, I don't know which glitches they were using before that are no longer valid. As far as I could tell, they just kept utilizing the Lv64 Trait's Shukuchi Reset and they were hard enough to outrun already, but now they have waaaay more access to that thanks to Shadowbringers giving them two charges on Shukuchi and two charges on Kassatsu. So it's only a matter of time before they blow every other class out of the water; walking into a dungeon as a Lv74 NIN should basically a free world record at this point lol