I should note before I go on. There have been two proposed ways to make Jab cancelling easier. The first is to hold diagonally backwards which allows you to Jab while crouching, and the other is to hold down with the joystick and hit diagonally down with the A-stick. Of these two methods, only the latter actually helps. If you use the diagonally down and backwards joystick method, you have to wait till Link has crouched all the way back down again before you are able to jab, otherwise a F-tilt backwards will come out. So you can think of this tech as the final nail in the coffin of the 'c-stick or a-stick' debate. I personally am going to switch to a-stick because of this. Also, if anyone is worried about this being too difficult, it's nothing compared to the Brawl Jab Lock. The Link mains have got this.
(Note that the a-stick does not work well in training mode slower speeds, so all of the following was done the old fashioned way with 1/4 hold, crouching and a lot of patience.)
Here's what I've tested so far in training mode.
In the video it said that Diddy starts at around 50% but if you're frame perfect, you can start it at 40%. At 39% Diddy will shield the next Jab. [50% is probably a more realistic percentage to go by of course because of human error, but I'm using 40% for testing purposes.]
I used this to test whether DI affects things. I had Diddy taunt and then hold down so that there would be DI downwards with no crouch cancelling affecting the knockback. I was still able to get it to work on 40% with Diddy unable to shield if Link did it perfectly. This is not too surprising seeing as DI isn't going to have much effect on such a small knockback move. A single SDI input downwards likewise seemed to have no effect.
I then tested to see how it works on all characters.
For now I'm just going to test starting percents. Note that the following percents assume that Jab is not stale and that Link is on the same percent.
Mario can double Jump out at 0%. Clearly it helps if they are forced to land. We'll be seeing a lot of this I'd wager. Actually, I'll just list all the characters who, along with Mario, can double jump to escape this at 0% right here.
Luigi, Peach, Yoshi, Rosalina, Bowser Jr (your DJ will get Jabbed if you don't immediately airdodge, and if you mess it up you're in a tough spot), Wario, DK, GnW can Up-B out (DJ gets Jabbed), Mac, Link, Zelda, Tink, Samus, Pit, Marth, Ike, Robin, DHD, Kirby, DDD, Pikachu (DJ gets Jabbed but Pika can Up-B after that to avoid the next Jab or just DJ immediately airdodge), Lucario, Jiggs, ROB (DJ gets Jabbed but ROB can Up-B after to avoid the next Jab or just DJ immediately airdodge), Ness (will get the DJ jabbed unless you immediately airdodge (if you mess it up you're screwed)), Villager, Olimar, WFT, Shulk (even in shield stance), Dr Mario, Dark Pit, Lucina, Pac-Man, Sonic (DJ will get Jabbed, so just DJ and immediately airdodge or Up-B out), Gunner/Sword (DJ will get Jabbed but you can Up-B after to get out or just DJ and airdodge immediately)
Edit: Note that there is another post I made further on that details what percent the Jab tech does work on the above characters.
Now I'll put all the interesting characters that this actually works on below with their starting percents. Note that the percents given are assuming that Link is frame perfect so you may want to add on like 10 to 20%.
-Bowser: Works from 0%
-Diddy: Works from 40%
-Sheik: Works from 64%
-Ganon: Works from 5%
-ZSS: Works from 47%
-Palutena: Works from 11%
-MK: Works from 28%
-Fox: Works from 62%
-Falco: Works from 62%
-Charizard: Works from 2%
-Greninja: Works from 67% (Greninja can just use Shadow Sneak to escape as it can be used during hitstun)
-C Falcon: Works from 58%
-Mega Man: Works from 27%
-Brawler: Works from 11%
Edited for Greninja update.