I’ve done some testing, and turns out you can buffer a spin from literally any state in the game. This includes bonking, jumping, crouching, going through a loading zone, rolling, etc. - as long as you end up in a state which allows you to spin, you can buffer the input at any time.

The biggest time save that I can see from this is from spin-jumping, then groundpounding and pressing Y as you land to start a roll at maximum speed. It’s also good when you need to make a high jump right after collecting a moon.

I’m not a speedrunner myself so I can’t say how useful this will be; I just noticed that it could be done and figured it could help.

How do you buffer that? Just spin the stick?

you probably just spin before you can move again

Just spin the stick and you spin on the first frame that the option becomes available.