I'm not sure if this is the sort of thing you're looking for, but I'll repost it, calling it "jab confirming" for now, from the original time I made it:
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This only works for certain characters, on whom holding A neither rapid fires the initial jab (like for Mario, Shulk, Sonic, Pikachu etc.) nor flows through the entire jab combo on whiff (Peach, Wario, Pit, etc.). It also doesn't work on characters with one-attack jabs (Zelda, Ganon)
What you do is hold A to perform your character's jab, then hold R after you start holding A
What happens is, while you're holding A, your character will jab: If it connected with a target, it will auto-transition into the next jab as usual; If it didn't, you will be holding R already, and will shield as soon as you can act out of your initial jab's animation
This is helpful because holding R will not interrupt your jab in the event that it hit-confirms on your enemies, allowing you to get your jab's full effect, but it will also allow you to interrupt your initial jab as soon as possible, should the jab miss. It doesn't simply have to be cancelled into shield per se, this also gives you the option of immediately sidestepping, rolling, or even jumping-out-of-shield right after your jab misses. You essentially have all your movement options the instant your shield comes up, and also the benefit of being able to use your shield to prevent taking damage. Holding R helps indicate when that moment happens
To reiterate, what's nice is that holding A in advance does not interfere with any of your shield-related options at all, while holding R in advance does not interfere with your character's jab being able to take its course
This essentially lets you throw out your character's jab--which is generally your character's safest move on hit, block, and whiff--and gain initiative if you hit, but also prevent opponent's from getting on the offensive if you miss. This is infinitely more safe than manually tapping A for a jab combo, or holding A without doing anything else, in the hopes that it will connect
Another nice thing about this is that it helps you get your timing down, if and when you want to go for a jab-cancel into whatever: Smash attack, grab, tilt attack, dash-grab, maybe even another jab, etc.
Possible:
Luigi
Bowser
Donkey Kong
Diddy Kong
Sheik
Falco
Lucario
G&W
Rosalina
Greninja
Robin
Palutena
DH
Gunner
Samus
Dedede
Marth/Lucina
ROB
Not possible:
Mario/Dr. Mario
Peach
Yoshi
Wario
Zelda
Ganon
ZSS
Kirby
MK
Fox
Pikachu
Jigglypuff
Charizard
CF
Ness
Ike
Pit/DP
Sonic
Bowser Jr.
LM
WFT
Shulk
Pacman
Brawler
Swordfighter
Link
Toon Link
Olimar
Villager
Megaman
For Samus and Marth/Lucina, their follow-up jab combo attacks are so slow and so unreliable that it's almost better to always to release the A button after the first jab happens, preventing their garbage jab "combos" from ever happening
For Dedede, his jab is unique, in that holding A will do the first two hits on whiff, so if holding R after holding A, he'll shield after that second hit. It's convenient that the auto-combo goes as far as the best part of his jab, but may make it questionably more unsafe on whiff, and might require more manual confirming
Both Link's and Olimar just perform their whole combo if A is held, regardless of hit or miss, swatting at the air
Megaman's jab is different, in that it only responds to repeated input, rather than held input
Villager simply can't do it; the jab just continues for as long as A is held
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