This old spikey tool gets rolled over your lawn and digs holes into it. These holes are to help the lawn 'breathe' and help the worms thrive better, again helping to improve the grass. Since my lawn is heavy with clay, breaking it up like this was something I wanted to try. Got the implement from the Wheatley Swap Shop held in the Merry Bells.
Found some references to this: www.anythingoes.net/Garden.html