"The Invisible foot" in this play is the foot of the market. Because you don't see the foot, it can trip you up. It usually trips you up when you are at your most confident, when the chance to make a financial gain is at its most potent. Like much in this clever and funny play, the foot is symbolic. It tells us that the market is cunning, illusive, and tricky. Just when we think we know it, it can hurl us to the floor and force us to think again.