It is just that Applejack's kick was so powerful that it caused an magical explosion. At least that's what I think that happened, I made that icon and the wallpaper that it is from, and I don't even get it myself.
The lines supposedly leaked implying Discord was Starswirl were confirmed as fake anyways. Discord is not Starswirl, and that's one of the dumbest things I've ever heard of. I'm not saying people can't speculate on Discord's origins. I tend to encourage creative thought like that. I'm saying that Celestia knew Starswirl personally, and it's not like Celestia to simply forget to mention to her own personal student that one of the most talented unicorns in history who she also idolizes is standing right in front of her. Discord may be a pain in the rear, but that would change Twilight's opinion of him pretty drastically. More to the point, if Discord was Starswirl, Celestia would have greeted him as an old friend when he was reformed. I'd also like to point this out: in the very same episode Discord says in his own words "because you're the only friend I've ever had". I think it's safe to say if Discord was Starswirl, then he would have seen Celestia as a friend when he knew her. There's too much evidence against the theory, and not a shred of canon evidence towards the theory.
On a related note, I'd like to bring this up to everyone who draws parallels between Starswirl and Merlin: Merlin's magic power came from the fact that he was a cambion; the son of a succubus. Bet you didn't know that detail of that story did you?
the only thing I 'knew' about Merlin (since I don't believe he or Arther actually existed) was that he had 2 crappy live-action movies (maybe more), and one funny Disney movie that mostly centered on child-Arther, and all versions of the legend states that he was an adviser and friend to king Arther. Anything else I may or may not have heard/read about him changes at least slightly from story to story. Not all of them say he could use magic (though most certainly do), and none of those that I have seen or read stated anything about his heritage, save one young-adults book that said (by his mother) he was part angel which was why his back was pained in place where wings would have been positioned.
And really, *I don't care*. I haven't even seen the second half of the 3'rd season of MLP yet (been too distracted with life to actually sit down and watch it all). I only posted what I posted because you stated " I'm pretty sure John De Lance knows his own character, and he tends to have loose lips." and I was poking a hole in that opinion.
Perhaps Starswirl was thinking that the spell could be used to switch the destinies of ponies who truly were unhappy with their talents. And a pony could use it it to change their destiny either for the greater good or some other reason.