I can’t be the only one thinking that she’s going to be a juggernaut when she grows up.
 
It was a secret everyone’s known since before she could form words in her mouth. Throughout her life as the heiress of the Crystal Empire, Flurry Heart always had several uncontrollable bursts of energy. But as it came, it always tends to disappear…until now. At last, the beast within the Princess has been harnessed. Flurry Heart has awoken.
 
As for how it happens:
 
Spike: (noticing Grogar have a sadistic look) Twilight, is Flurry even AWARE of your plan to let her try to stop Grogar on her own?  
Twilight: (shocked) What do you mean?  
Spike: ARE YOU KIDDING ME!?! She may be a bit of a brat on occasion, but this is a foal who hasn’t even had her Cutie Mark spawn, and you’re having her face THAT thing? I don’t care if Celestia taught you the same way. The important thing is that Flurry Heart is not YOU. And I don’t care if you stop me. I’d rather DIE than wait!
 
As for how she freaking unleashes it…well…let’s just say that he would have gone the extra mile just to kick the dog when he’s down.
 
After a moment, well…the scream might be heard…and I picture this being heard in the background.