Instead of playing it and seeing what he's doing (because I can't - look at previous post), I decided to just go and look at the dsm, and I'm 99% sure I found where it happens (look
here to see what's happening):
|0|......B.Y....000 000 0|
|0|......B.Y....000 000 0|
|0|......B.Y....000 000 0|
|0|......B.Y....000 000 0|
|0|........Y....000 000 0|
|0|........Y....000 000 0|
|0|........Y....000 000 0|
|0|........Y....000 000 0|
|0|........Y....000 000 0|
|0|R............000 000 0|
|0|R.......Y....000 000 0|
|0|R.......Y....000 000 0|
|0|R.......Y....000 000 0|
|0|R.......Y....000 000 0|
|0|R.......Y....000 000 0|
The line without the Y is where he goes out of the shell, and then he somehow does a double jump by just repressing Y two frames later??? Maybe pressing 'right' while not having Y pressed or letting go of B before letting go of Y has something to do? Not completely sure how I would test that (but at least we're making progress!).