Found at this other blog.

Quoting the original article:

This unexpected, spontaneous, and scripted dance (aka Flash dance) was performed by over twenty thousand people. How did they do it?

1. Planners recruited 800 people using Facebook and Twitter
2. Choreographers taught a simple dance to these 800 “dancers”
3. Dancers were planted amongst the crowd, who then the dance to the people around them
4. Music started playing, people started dancing.

One person doing the dance is funny. 100 people in the front of the stage doing the dance is entertaining. Everybody doing the dance in unison is amazing.