I hereby dub this the Sparkling Rainboom.

Sonic raingasm.

@James Rye  
Yes, this needs to be written.

And even here Twilight’s going to crash-land.

@JP
 
Now we only need a shipfic with it as cover. Kinda like Rainbow carrying Twi in her arms, you know? <3  
Like both hug each other very strongly while RD takes more and more speed, heats beating faster, she warns Twi just at the right time, the purple alicorn charges her horn with magic, and the moment RD BREAKS through the sound barrier with A BOOM Twi uses her spell to create a BURST of magical purple star surrounded by RD’s rainbow spreading circle and both flew further away, looking behind them, seeing their glorious creation of epicness, then they look smiling int each other’s eyes and kiss tenderly with a blush while flying at Mach 1……dang, I wanna read a shipfic about it now! D:

@Background Pony #739D
 
Well, we see her horn glowing, so it’s probably just some kind of magical firework spell.

@Background Pony #739D  
According to Meghan McCarthy, that boom in the actual episode was just magic.
 
This picture however might be different; I can’t speak for the artist.

@Background Pony #739D  
No, she just used magic to make an effect.

Was that really Twilight’s Sonic Rainboom? She barley got the hang of staying airborne.

Exhibit A: Why TwiDash is the best ship.