...when they do 'tumbling', which is lots of back flips and forward flips etc all in a row. I've spent part of the afternoon walking on their bouncy tumbling track, finally succoming to running up and down it with a friend. Apart from dodging small children it was fantastic. Sort of like running on springs, or maybe the moon.
Highly recommended as a Friday afternoon activity. And have no idea what the people not playing running thought of us, and even better only care slightly.