This practice is designed to provide a crowdsourced approach to problem solving. The problem is posed to the group by 'the fly' and the group then discusses the problem without any interruption from 'the fly'.
All too often we can blind ourselves to a solution to a complex problem we face. "I've tried that", "Ahh, but that won't work because..." These are common responses to solutions suggestions from other people; we cut off the idea at the knees before it has been formed. Through this practice, the 'fly on the wall' must listen and let the conversation flow. They must remain curious and let the ideas fully form before being brought back into the conversation to feedback and add any insight/ commentary upon what has been said.
This particular practice is suited to teams that are working primarily as individuals or pairs rather than those practiced in a 'mob to learn, pair to build' approach. It's a great way to demonstrate the power of group think and inclusive problem solving.
This is a time boxed exercise.
Check out these great links which can help you dive a little deeper into running the Fly on the Wall practice with your team, customers or stakeholders.