What is a Penny Drop? What does Will tell the reader about a Penny Drop in Chapter One?

A Penny Drop is being on the monkey bars upside and swinging back and forth until releasing the legs and landing with feet on the ground if timed correctly. Will tells the reader that Shawn taught Will how to do a Penny Drop after Shawn learns it from Riggs and that being able to do a Penny Drop or a backflip meant being the king of the playground.