Simply put - the person who places the marker at the furthest point wins the competition. Weight is obviously the key here so this is the one place where all those pizzas and beer consumed over the years has benefit. So if you did not do all that well on the rest of the activities - this is the time to strut your stuff!
Of course speed can overcome weight so at times it's a competition between the speedsters and the rugby forwards and this is what makes it interesting.
Works great for just families having a go or especially for team challenges. It's a heck of a lot of fun but be careful - once you've planted the marker - dig in or you can get flung all the way back to where the ducks nest.