Sonic Rainboom

Rainbow Dash's signature ability.

Rainbow Dash can only produce a Sonic Rainboom when being extraordinarily loyal

When Pinkie Pie was a filly on the rock farm, she imagined a Sonic Rainboom filling the sky, and what kind of magical world it could come from. This imaginary world of hers is Equestria as we know it.

A Sonic Rainboom is impossible to perform for anypony other than Rainbow Dash.

A Sonic Rainboom is made when Rainbow Dash breaks the rainboom barrier, not the sound barrier.

Breaking the sound barrier isn't enough to do a Sonic Rainboom. It has something to do with Pegasi Magic.