I didn't have the benefit of being in a collaborative story-making event like the ones he described, where someone else provides you a prompt over which you have no control, but I realized I had something similar. A couple of months ago, I was watching the super super-moon with a second grader friend of mine, when something she did or said made me suddenly picture a girl reaching up and pulling the moon out of the sky. I didn't think any more about it, but the image stuck in my mind, and now I knew what use I could make of it.
I began writing that scene, a girl grabbing the moon, and as I wrote, a story began to take shape. I'm afraid I only ended up writing about half of it, because I reached a point where I'll need to research what might actually happen on the Earth if the Moon were to suddenly disappear. (How would it affect the tides? Would it confused any types of animals that depend on it for navigation? etc.) But I'm satisfied that the prompt did its work, and that there is a story there to be finished.
Here's my favorite bit of it so far:
She had finally, really, amazingly, impossibly caught the moon!
She looked up into the sky again. The moon was still not there. She looked into her hand, and there it still was. Its light felt cool on her face.