There is tension and excitement since the crowds do not know what is happening. Is the figure on top of the tall building about to jump to his death? Might he (or she) accidentally fall? Eventually, the observers figure out what is happening. They see the wire and the balancing pole the walker has in his hands.  There is still fear, but then amazement and awe as the tightrope walker performs his act, including hopping and lying down on the wire. 

The author captures the mob psychology of the event with some in the crowd hoping to see a spectacular fall and tragic death and others hoping to see the magic of a seemingly impossible performance.