I just wanted to know since i wanted to add it into my rom hack, that's all

u need to know how to code in asm by, making the physics and animation for it and trust, its really challenging if u know nothing, so i wouldn't go for it.

EDIT: apparantly just found out someone (IllyCat) said they posted a patch for a spinjump in nsmbc discord. I dont use discord but u can check.
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For that, you'll need to know how to set up the code hack patching template (with NCPatcher), and patch code hacks.

There's a midair twirl code hack that has been uploaded to a few places, made by IllyCat and other people, which you can find here for example https://github.com/NSMBC/Code-Mods/blob/main/Player/Movement/midairtwirl.cpp

As for NCPatcher patching template, you can find it here https://github.com/MammaMiaTeam/NSMB-Code-Template. Download by clicking "Code" then "Download ZIP" The github should explain everything, though it can still be complicated. If you need help you can also ask!!

I hope this helps ^-^
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Currently working on:
New Super Mario Bros. The Rainbow Quest

Our Rom hacking Youtube channel @RainbowDashers64

On second thought maybe I won't since code patching requires God knows how many steps.