Others were playing at leapfrog, or chasing each other from pillar to post.
Leapfrog is one of his methods of getting over the ground quickly.
One was leapfrog, and another was of the snap-the-whip variety.
It is not necessary to be able to play at leapfrog to enjoy the game.
Mr. Miller had risen and bowed as if he was giving some invisible person a “back” for leapfrog.