It's one of her secret moves, so tbat means it need s a special motion to be inputted.
in this case, hold left or right, away from your opponent for about one second. Then quickly press forward + attack. You can see if you hold away long enough if hearts start appearing around shella snail.
You can also hold (left or right) away while crouching or attacking. This ensures that you stand your ground while charging. This also means that you will lose your heart charge once you start walking forward again.
I'm planning on simplifying this a bit in the next update. It may be a bit confusing at first, but I think you'll get the hang of it.