The concept is simple. One dancer dances in front of the camera until the bass drops and ‘do the Harlem shake’ is announced. Then the camera often cuts to a more obscure form of dancing that shocks and entertains viewers.
In the Firefighter Edition, one fighter dances while two quietly sit. Until the bassdrop. Then the camera cuts to one extra in a Spiderman costume going nuts, and the other flipping out in a chicken suit.
Ye, this is what the web was made for.