Are you talking about during the Bowser fight?

It's not a very easy trick to get the first couple times you attempt it. I've tried it myself, and it's pretty hard. (Granted I've only beat the game a couple times total.)

But it seems that jumping into bowsers face (not touching him) is what causes the second fireball animation to be completely skipped (for some unknown reason). It has something to do with being in the right place at the right time. And can only be done with the second fireball that he shoots (not the first).

When Ewaller ran NSMB U at SGDQ, he was able to explain it slightly better I think (it effects both games).

Hope this helps.

yeah i use bowser jr car flying in the sky to time my jump, and just spin jump then boost. seems to basically trigger him swiping at you probably from getting too close to his face, and then bowser jr will come right down

This was the only thing I discovered in the game :/ most of the strats ewaller, pokey, and spades found.