Скачать или смотреть Aerial Boosted Rising Uppercut - Little Mac Advanced Tech - Super Smash Bros Ultimate

explanation of how/why

Mac's jump animations will cause up b to not move forward. Otherwise this "boosted" up b can be seen normally if you manage to be in the air long enough, out of tumble, and other animations. By using an aerial you immediately cut out the standard jump animation.

Why would they tie it to the jump animations? During the first 3 frames of up b his fall speed affects how far he moves in those frames. Ie; you fast fall - he will drop down low during those frames of up b. So in order to easily nerf his recovery- the devs seems to just do a quick patch job to prevent a double jump boost on his up b. Some characters can do a similar thing and use doublejump inertia to boost their recovery to an extent. So smash devs quickly slapped his jump animations to prevent a decent recovery and shipped the game out