Mysmashcorner made a video about this, but in order to do this method, you'd either have to claw grip or set one shoulder button to jump.
https://www.youtube.com/watch?v=v1kjQtMaPkE skip to 6min 20sec
Press forward on the control stick to start your initial dash and hold this direction during the entire technique
at any point during your initial dash animation flick the c-stick in the opposite direction
press jump after flicking the c-stick
Your character will perform a Bair on the first possible frame while preserving your forward momentum. Also, you won't have to go through your entire initial dash, unlike a normal RAR Bair. I'm not sure how Nairo does it but that's what I use for characters that can take advantage of it. Inkling can get Fthrow>Bair, Fox can mix Bair into his combo trees, and Roy/Chrom can do Jab>Bair. It's not very difficult to do either.