Love....
...the power of love...
...friendship IS love.
It's the strongest, most powerful, greatest kind of love of all.
Applejack and Rainbow Dash are friends.
That 's why they have the power of love.
Cadence didn't do anything, the power of love was always within them the whole time.
It's within all of the Mane Six. It's what gives the Elements of Harmony their power, what creates the Holy Rainbow and drives away the darkness.
The power the six Ponies have.
The power of friendship.
The power of love.