Rarest of the rare

"We all move forwards dear."

"Yes, granny" Sighed Jake. Granny had her explaining voice on again.

"There are those rare ones who can move side-to-side. And then there are the rarest of the rare ones. The special ones..." Jake pricked his ears.

"What special one's granny?"

"They are so rare! One in a billion. There are likely only six or seven in the world out of everyone!"

"So why are they rare granny?"

"Oh Ho! They can do something that no-one else can do. They can move backwards!"

"I don't understand granny."

"That's alright boy. You will..."